Merge branch 'wip-phpstan-level6'

49 files changed, 2409 insertions, 943 deletions

diff --git a/classes/api.php b/classes/api.php
index 033aa8654..764cb04da 100755
--- a/classes/api.php
+++ b/classes/api.php
@@ -13,13 +13,20 @@ class API extends Handler {
 	const E_UNKNOWN_METHOD = "UNKNOWN_METHOD";
 	const E_OPERATION_FAILED = "E_OPERATION_FAILED";
 
+	/** @var int|null */
 	private $seq;
 
-	private static function _param_to_bool($p) {
+	/**
+	 * @param mixed $p
+	 */
+	private static function _param_to_bool($p): bool {
 		return $p && ($p !== "f" && $p !== "false");
 	}
 
-	private function _wrap($status, $reply) {
+	/**
+	 * @param array<int|string, mixed> $reply
+	 */
+	private function _wrap(int $status, array $reply): void {
 		print json_encode([
 					"seq" => $this->seq,
 					"status" => $status,
@@ -27,7 +34,7 @@ class API extends Handler {
 				]);
 	}
 
-	function before($method) {
+	function before(string $method): bool {
 		if (parent::before($method)) {
 			header("Content-Type: text/json");
 
@@ -48,17 +55,17 @@ class API extends Handler {
 		return false;
 	}
 
-	function getVersion() {
+	function getVersion(): void {
 		$rv = array("version" => Config::get_version());
 		$this->_wrap(self::STATUS_OK, $rv);
 	}
 
-	function getApiLevel() {
+	function getApiLevel(): void {
 		$rv = array("level" => self::API_LEVEL);
 		$this->_wrap(self::STATUS_OK, $rv);
 	}
 
-	function login() {
+	function login(): void {
 
 		if (session_status() == PHP_SESSION_ACTIVE) {
 			session_destroy();
@@ -87,22 +94,20 @@ class API extends Handler {
 			} else {
 				$this->_wrap(self::STATUS_ERR, array("error" => self::E_API_DISABLED));
 			}
-		} else {
-			$this->_wrap(self::STATUS_ERR, array("error" => self::E_LOGIN_ERROR));
-			return;
 		}
+		$this->_wrap(self::STATUS_ERR, array("error" => self::E_LOGIN_ERROR));
 	}
 
-	function logout() {
+	function logout(): void {
 		UserHelper::logout();
 		$this->_wrap(self::STATUS_OK, array("status" => "OK"));
 	}
 
-	function isLoggedIn() {
+	function isLoggedIn(): void {
 		$this->_wrap(self::STATUS_OK, array("status" => $_SESSION["uid"] != ''));
 	}
 
-	function getUnread() {
+	function getUnread(): void {
 		$feed_id = clean($_REQUEST["feed_id"] ?? "");
 		$is_cat = clean($_REQUEST["is_cat"] ?? "");
 
@@ -114,12 +119,12 @@ class API extends Handler {
 	}
 
 	/* Method added for ttrss-reader for Android */
-	function getCounters() {
+	function getCounters(): void {
 		$this->_wrap(self::STATUS_OK, Counters::get_all());
 	}
 
-	function getFeeds() {
-		$cat_id = clean($_REQUEST["cat_id"]);
+	function getFeeds(): void {
+		$cat_id = (int) clean($_REQUEST["cat_id"]);
 		$unread_only = self::_param_to_bool(clean($_REQUEST["unread_only"] ?? 0));
 		$limit = (int) clean($_REQUEST["limit"] ?? 0);
 		$offset = (int) clean($_REQUEST["offset"] ?? 0);
@@ -130,7 +135,7 @@ class API extends Handler {
 		$this->_wrap(self::STATUS_OK, $feeds);
 	}
 
-	function getCategories() {
+	function getCategories(): void {
 		$unread_only = self::_param_to_bool(clean($_REQUEST["unread_only"] ?? false));
 		$enable_nested = self::_param_to_bool(clean($_REQUEST["enable_nested"] ?? false));
 		$include_empty = self::_param_to_bool(clean($_REQUEST['include_empty'] ?? false));
@@ -186,11 +191,11 @@ class API extends Handler {
 		$this->_wrap(self::STATUS_OK, $cats);
 	}
 
-	function getHeadlines() {
+	function getHeadlines(): void {
 		$feed_id = clean($_REQUEST["feed_id"]);
-		if ($feed_id !== "") {
+		if ($feed_id !== "" && is_numeric($feed_id)) {
 
-			if (is_numeric($feed_id)) $feed_id = (int) $feed_id;
+			$feed_id = (int) $feed_id;
 
 			$limit = (int)clean($_REQUEST["limit"] ?? 0 );
 
@@ -237,7 +242,7 @@ class API extends Handler {
 		}
 	}
 
-	function updateArticle() {
+	function updateArticle(): void {
 		$article_ids = explode(",", clean($_REQUEST["article_ids"]));
 		$mode = (int) clean($_REQUEST["mode"]);
 		$data = clean($_REQUEST["data"] ?? "");
@@ -303,7 +308,7 @@ class API extends Handler {
 
 	}
 
-	function getArticle() {
+	function getArticle(): void {
 		$article_ids = explode(',', clean($_REQUEST['article_id'] ?? ''));
 		$sanitize_content = self::_param_to_bool($_REQUEST['sanitize'] ?? true);
 
@@ -351,7 +356,7 @@ class API extends Handler {
 					$article['content'] = Sanitizer::sanitize(
 						$entry->content,
 						self::_param_to_bool($entry->hide_images),
-						false, $entry->site_url, false, $entry->id);
+						null, $entry->site_url, null, $entry->id);
 				} else {
 					$article['content'] = $entry->content;
 				}
@@ -375,7 +380,10 @@ class API extends Handler {
 		}
 	}
 
-	private function _get_config() {
+	/**
+	 * @return array<string, array<string, string>|bool|int|string>
+	 */
+	private function _get_config(): array {
 		$config = [
 			"icons_dir" => Config::get(Config::ICONS_DIR),
 			"icons_url" => Config::get(Config::ICONS_URL)
@@ -391,13 +399,13 @@ class API extends Handler {
 		return $config;
 	}
 
-	function getConfig() {
+	function getConfig(): void {
 		$config = $this->_get_config();
 
 		$this->_wrap(self::STATUS_OK, $config);
 	}
 
-	function updateFeed() {
+	function updateFeed(): void {
 		$feed_id = (int) clean($_REQUEST["feed_id"]);
 
 		if (!ini_get("open_basedir")) {
@@ -407,10 +415,10 @@ class API extends Handler {
 		$this->_wrap(self::STATUS_OK, array("status" => "OK"));
 	}
 
-	function catchupFeed() {
+	function catchupFeed(): void {
 		$feed_id = clean($_REQUEST["feed_id"]);
 		$is_cat = clean($_REQUEST["is_cat"]);
-		@$mode = clean($_REQUEST["mode"]);
+		$mode = clean($_REQUEST["mode"] ?? "");
 
 		if (!in_array($mode, ["all", "1day", "1week", "2week"]))
 			$mode = "all";
@@ -420,13 +428,13 @@ class API extends Handler {
 		$this->_wrap(self::STATUS_OK, array("status" => "OK"));
 	}
 
-	function getPref() {
+	function getPref(): void {
 		$pref_name = clean($_REQUEST["pref_name"]);
 
 		$this->_wrap(self::STATUS_OK, array("value" => get_pref($pref_name)));
 	}
 
-	function getLabels() {
+	function getLabels(): void {
 		$article_id = (int)clean($_REQUEST['article_id'] ?? -1);
 
 		$rv = [];
@@ -462,7 +470,7 @@ class API extends Handler {
 		$this->_wrap(self::STATUS_OK, $rv);
 	}
 
-	function setArticleLabel() {
+	function setArticleLabel(): void {
 
 		$article_ids = explode(",", clean($_REQUEST["article_ids"]));
 		$label_id = (int) clean($_REQUEST['label_id']);
@@ -477,9 +485,9 @@ class API extends Handler {
 			foreach ($article_ids as $id) {
 
 				if ($assign)
-					Labels::add_article($id, $label, $_SESSION["uid"]);
+					Labels::add_article((int)$id, $label, $_SESSION["uid"]);
 				else
-					Labels::remove_article($id, $label, $_SESSION["uid"]);
+					Labels::remove_article((int)$id, $label, $_SESSION["uid"]);
 
 				++$num_updated;
 
@@ -491,7 +499,7 @@ class API extends Handler {
 
 	}
 
-	function index($method) {
+	function index(string $method): void {
 		$plugin = PluginHost::getInstance()->get_api_method(strtolower($method));
 
 		if ($plugin && method_exists($plugin, $method)) {
@@ -504,7 +512,7 @@ class API extends Handler {
 		}
 	}
 
-	function shareToPublished() {
+	function shareToPublished(): void {
 		$title = strip_tags(clean($_REQUEST["title"]));
 		$url = strip_tags(clean($_REQUEST["url"]));
 		$content = strip_tags(clean($_REQUEST["content"]));
@@ -516,13 +524,12 @@ class API extends Handler {
 		}
 	}
 
-	private static function _api_get_feeds($cat_id, $unread_only, $limit, $offset, $include_nested = false) {
+	/**
+	 * @return array<int, array{'id': int, 'title': string, 'unread': int, 'cat_id': int}>
+	 */
+	private static function _api_get_feeds(int $cat_id, bool $unread_only, int $limit, int $offset, bool $include_nested = false): array {
 			$feeds = [];
 
-			$limit = (int) $limit;
-			$offset = (int) $offset;
-			$cat_id = (int) $cat_id;
-
 			/* Labels */
 
 			/* API only: -4 All feeds, including virtual feeds */
@@ -632,13 +639,16 @@ class API extends Handler {
 		return $feeds;
 	}
 
-	private static function _api_get_headlines($feed_id, $limit, $offset,
-				$filter, $is_cat, $show_excerpt, $show_content, $view_mode, $order,
-				$include_attachments, $since_id,
-				$search = "", $include_nested = false, $sanitize_content = true,
-				$force_update = false, $excerpt_length = 100, $check_first_id = false, $skip_first_id_check = false) {
+	/**
+	 * @return array{0: array<int, array<string, mixed>>, 1: array<string, mixed>} $headlines, $headlines_header
+	 */
+	private static function _api_get_headlines(int $feed_id, int $limit, int $offset,
+				string $filter, bool $is_cat, bool $show_excerpt, bool $show_content, ?string $view_mode, string $order,
+				bool $include_attachments, int $since_id, string $search = "", bool $include_nested = false,
+				bool $sanitize_content = true, bool $force_update = false, int $excerpt_length = 100, ?int $check_first_id = null,
+				bool $skip_first_id_check = false): array {
 
-			if ($force_update && $feed_id > 0 && is_numeric($feed_id)) {
+			if ($force_update && is_numeric($feed_id) && $feed_id > 0) {
 				// Update the feed if required with some basic flood control
 
 				$feed = ORM::for_table('ttrss_feeds')
@@ -746,7 +756,7 @@ class API extends Handler {
 							$headline_row["content"] = Sanitizer::sanitize(
 								$line["content"],
 								self::_param_to_bool($line['hide_images']),
-								false, $line["site_url"], false, $line["id"]);
+								null, $line["site_url"], null, $line["id"]);
 						} else {
 							$headline_row["content"] = $line["content"];
 						}
@@ -803,7 +813,7 @@ class API extends Handler {
 			return array($headlines, $headlines_header);
 	}
 
-	function unsubscribeFeed() {
+	function unsubscribeFeed(): void {
 		$feed_id = (int) clean($_REQUEST["feed_id"]);
 
 		$feed_exists = ORM::for_table('ttrss_feeds')
@@ -818,7 +828,7 @@ class API extends Handler {
 		}
 	}
 
-	function subscribeToFeed() {
+	function subscribeToFeed(): void {
 		$feed_url = clean($_REQUEST["feed_url"]);
 		$category_id = (int) clean($_REQUEST["category_id"]);
 		$login = clean($_REQUEST["login"]);
@@ -833,7 +843,7 @@ class API extends Handler {
 		}
 	}
 
-	function getFeedTree() {
+	function getFeedTree(): void {
 		$include_empty = self::_param_to_bool(clean($_REQUEST['include_empty']));
 
 		$pf = new Pref_Feeds($_REQUEST);
@@ -846,7 +856,7 @@ class API extends Handler {
 	}
 
 	// only works for labels or uncategorized for the time being
-	private function _is_cat_empty($id) {
+	private function _is_cat_empty(int $id): bool {
 		if ($id == -2) {
 			$label_count = ORM::for_table('ttrss_labels2')
 				->where('owner_uid', $_SESSION['uid'])
@@ -865,7 +875,8 @@ class API extends Handler {
 		return false;
 	}
 
-	private function _get_custom_sort_types() {
+	/** @return array<string, string>  */
+	private function _get_custom_sort_types(): array {
 		$ret = [];
 
 		PluginHost::getInstance()->run_hooks_callback(PluginHost::HOOK_HEADLINES_CUSTOM_SORT_MAP, function ($result) use (&$ret) {
diff --git a/classes/article.php b/classes/article.php
index 04855ac9d..e113ed219 100755
--- a/classes/article.php
+++ b/classes/article.php
@@ -4,7 +4,11 @@ class Article extends Handler_Protected {
 	const ARTICLE_KIND_VIDEO = 2;
 	const ARTICLE_KIND_YOUTUBE = 3;
 
-	function redirect() {
+	const CATCHUP_MODE_MARK_AS_READ = 0;
+	const CATCHUP_MODE_MARK_AS_UNREAD = 1;
+	const CATCHUP_MODE_TOGGLE = 2;
+
+	function redirect(): void {
 		$article = ORM::for_table('ttrss_entries')
 			->table_alias('e')
 			->join('ttrss_user_entries', [ 'ref_id', '=', 'e.id'], 'ue')
@@ -24,8 +28,7 @@ class Article extends Handler_Protected {
 		print "Article not found or has an empty URL.";
 	}
 
-	static function _create_published_article($title, $url, $content, $labels_str,
-			$owner_uid) {
+	static function _create_published_article(string $title, string $url, string $content, string $labels_str, int $owner_uid): bool {
 
 		$guid = 'SHA1:' . sha1("ttshared:" . $url . $owner_uid); // include owner_uid to prevent global GUID clash
 
@@ -158,14 +161,14 @@ class Article extends Handler_Protected {
 		return $rc;
 	}
 
-	function printArticleTags() {
+	function printArticleTags(): void {
 		$id = (int) clean($_REQUEST['id'] ?? 0);
 
 		print json_encode(["id" => $id,
 			"tags" => self::_get_tags($id)]);
 	}
 
-	function setScore() {
+	function setScore(): void {
 		$ids = array_map("intval", clean($_REQUEST['ids'] ?? []));
 		$score = (int)clean($_REQUEST['score']);
 
@@ -179,14 +182,14 @@ class Article extends Handler_Protected {
 		print json_encode(["id" => $ids, "score" => $score]);
 	}
 
-	function setArticleTags() {
+	function setArticleTags(): void {
 
 		$id = clean($_REQUEST["id"]);
 
 		//$tags_str = clean($_REQUEST["tags_str"]);
 		//$tags = array_unique(array_map('trim', explode(",", $tags_str)));
 
-		$tags = FeedItem_Common::normalize_categories(explode(",", clean($_REQUEST["tags_str"])));
+		$tags = FeedItem_Common::normalize_categories(explode(",", clean($_REQUEST["tags_str"] ?? "")));
 
 		$this->pdo->beginTransaction();
 
@@ -254,18 +257,18 @@ class Article extends Handler_Protected {
 		print "</ul>";
 	}*/
 
-	function assigntolabel() {
-		return $this->_label_ops(true);
+	function assigntolabel(): void {
+		$this->_label_ops(true);
 	}
 
-	function removefromlabel() {
-		return $this->_label_ops(false);
+	function removefromlabel(): void {
+		$this->_label_ops(false);
 	}
 
-	private function _label_ops($assign) {
+	private function _label_ops(bool $assign): void {
 		$reply = array();
 
-		$ids = explode(",", clean($_REQUEST["ids"]));
+		$ids = array_map("intval", array_filter(explode(",", clean($_REQUEST["ids"] ?? "")), "strlen"));
 		$label_id = clean($_REQUEST["lid"]);
 
 		$label = Labels::find_caption($label_id, $_SESSION["uid"]);
@@ -289,11 +292,10 @@ class Article extends Handler_Protected {
 		print json_encode($reply);
 	}
 
-	static function _format_enclosures($id,
-										$always_display_enclosures,
-									   $article_content,
-										$hide_images = false) {
-
+	/**
+	 * @return array{'formatted': string, 'entries': array<int, array<string, mixed>>}
+	 */
+	static function _format_enclosures(int $id, bool $always_display_enclosures, string $article_content, bool $hide_images = false): array {
 		$enclosures = self::_get_enclosures($id);
 		$enclosures_formatted = "";
 
@@ -366,7 +368,10 @@ class Article extends Handler_Protected {
 		return $rv;
 	}
 
-	static function _get_tags($id, $owner_uid = 0, $tag_cache = false) {
+	/**
+	 * @return array<int, string>
+	 */
+	static function _get_tags(int $id, int $owner_uid = 0, ?string $tag_cache = null): array {
 
 		$a_id = $id;
 
@@ -383,12 +388,14 @@ class Article extends Handler_Protected {
 
 		/* check cache first */
 
-		if ($tag_cache === false) {
+		if (!$tag_cache) {
 			$csth = $pdo->prepare("SELECT tag_cache FROM ttrss_user_entries
 				WHERE ref_id = ? AND owner_uid = ?");
 			$csth->execute([$id, $owner_uid]);
 
-			if ($row = $csth->fetch()) $tag_cache = $row["tag_cache"];
+			if ($row = $csth->fetch()) {
+				$tag_cache = $row["tag_cache"];
+			}
 		}
 
 		if ($tag_cache) {
@@ -416,7 +423,7 @@ class Article extends Handler_Protected {
 		return $tags;
 	}
 
-	function getmetadatabyid() {
+	function getmetadatabyid(): void {
 		$article = ORM::for_table('ttrss_entries')
 			->join('ttrss_user_entries', ['ref_id', '=', 'id'], 'ue')
 			->where('ue.owner_uid', $_SESSION['uid'])
@@ -429,7 +436,10 @@ class Article extends Handler_Protected {
 		}
 	}
 
-	static function _get_enclosures($id) {
+	/**
+	 * @return array<int, array<string, mixed>>
+	 */
+	static function _get_enclosures(int $id): array {
 		$encs = ORM::for_table('ttrss_enclosures')
 			->where('post_id', $id)
 			->find_many();
@@ -452,7 +462,7 @@ class Article extends Handler_Protected {
 
 	}
 
-	static function _purge_orphans() {
+	static function _purge_orphans(): void {
 
         // purge orphaned posts in main content table
 
@@ -471,7 +481,11 @@ class Article extends Handler_Protected {
         }
     }
 
-	static function _catchup_by_id($ids, $cmode, $owner_uid = false) {
+	/**
+	 * @param array<int, int> $ids
+	 * @param int $cmode Article::CATCHUP_MODE_*
+	 */
+	static function _catchup_by_id($ids, int $cmode, ?int $owner_uid = null): void {
 
 		if (!$owner_uid) $owner_uid = $_SESSION["uid"];
 
@@ -479,11 +493,11 @@ class Article extends Handler_Protected {
 
 		$ids_qmarks = arr_qmarks($ids);
 
-		if ($cmode == 1) {
+		if ($cmode == Article::CATCHUP_MODE_MARK_AS_UNREAD) {
 			$sth = $pdo->prepare("UPDATE ttrss_user_entries SET
 				unread = true
 					WHERE ref_id IN ($ids_qmarks) AND owner_uid = ?");
-		} else if ($cmode == 2) {
+		} else if ($cmode == Article::CATCHUP_MODE_TOGGLE) {
 			$sth = $pdo->prepare("UPDATE ttrss_user_entries SET
 				unread = NOT unread,last_read = NOW()
 					WHERE ref_id IN ($ids_qmarks) AND owner_uid = ?");
@@ -496,7 +510,10 @@ class Article extends Handler_Protected {
 		$sth->execute(array_merge($ids, [$owner_uid]));
 	}
 
-	static function _get_labels($id, $owner_uid = false) {
+	/**
+	 * @return array<int, array<int, int|string>>
+	 */
+	static function _get_labels(int $id, ?int $owner_uid = null): array {
 		$rv = array();
 
 		if (!$owner_uid) $owner_uid = $_SESSION["uid"];
@@ -536,6 +553,8 @@ class Article extends Handler_Protected {
 		}
 
 		if (count($rv) > 0)
+			// PHPStan has issues with the shape of $rv for some reason (array vs non-empty-array).
+			// @phpstan-ignore-next-line
 			Labels::update_cache($owner_uid, $id, $rv);
 		else
 			Labels::update_cache($owner_uid, $id, array("no-labels" => 1));
@@ -543,6 +562,12 @@ class Article extends Handler_Protected {
 		return $rv;
 	}
 
+	/**
+	 * @param array<int, array<string, mixed>> $enclosures
+	 * @param array<string, mixed> $headline
+	 *
+	 * @return array<int, Article::ARTICLE_KIND_*|string>
+	 */
 	static function _get_image(array $enclosures, string $content, string $site_url, array $headline) {
 
 		$article_image = "";
@@ -577,6 +602,7 @@ class Article extends Handler_Protected {
 					} else if ($e->nodeName == "video") {
 						$article_image = $e->getAttribute("poster");
 
+						/** @var DOMElement|null $src */
 						$src = $tmpxpath->query("//source[@src]", $e)->item(0);
 
 						if ($src) {
@@ -603,14 +629,14 @@ class Article extends Handler_Protected {
 				}
 
 			if ($article_image) {
-				$article_image = rewrite_relative_url($site_url, $article_image);
+				$article_image = UrlHelper::rewrite_relative($site_url, $article_image);
 
 				if (!$article_kind && (count($enclosures) > 1 || (isset($elems) && $elems->length > 1)))
 					$article_kind = Article::ARTICLE_KIND_ALBUM;
 			}
 
 			if ($article_stream)
-				$article_stream = rewrite_relative_url($site_url, $article_stream);
+				$article_stream = UrlHelper::rewrite_relative($site_url, $article_stream);
 		}
 
 		$cache = new DiskCache("images");
@@ -624,7 +650,12 @@ class Article extends Handler_Protected {
 		return [$article_image, $article_stream, $article_kind];
 	}
 
-	// only cached, returns label ids (not label feed ids)
+	/**
+	 * only cached, returns label ids (not label feed ids)
+	 *
+	 * @param array<int, int> $article_ids
+	 * @return array<int, int>
+	 */
 	static function _labels_of(array $article_ids) {
 		if (count($article_ids) == 0)
 			return [];
@@ -651,6 +682,10 @@ class Article extends Handler_Protected {
 		return array_unique($rv);
 	}
 
+	/**
+	 * @param array<int, int> $article_ids
+	 * @return array<int, int>
+	 */
 	static function _feeds_of(array $article_ids) {
 		if (count($article_ids) == 0)
 			return [];
diff --git a/classes/auth/base.php b/classes/auth/base.php
index 82ea06e1b..d8128400d 100644
--- a/classes/auth/base.php
+++ b/classes/auth/base.php
@@ -8,13 +8,18 @@ abstract class Auth_Base extends Plugin implements IAuthModule {
 		$this->pdo = Db::pdo();
 	}
 
-	// compatibility wrapper, because of how pluginhost works (hook name == method name)
-	function hook_auth_user(...$args) {
-		return $this->authenticate(...$args);
+	function hook_auth_user($login, $password, $service = '') {
+		return $this->authenticate($login, $password, $service);
 	}
 
-	// Auto-creates specified user if allowed by system configuration
-	// Can be used instead of find_user_by_login() by external auth modules
+	/** Auto-creates specified user if allowed by system configuration.
+	 * Can be used instead of find_user_by_login() by external auth modules
+	 * @param string $login
+	 * @param string|false $password
+	 * @return null|int
+	 * @throws Exception
+	 * @throws PDOException
+	 */
 	function auto_create_user(string $login, $password = false) {
 		if ($login && Config::get(Config::AUTH_AUTO_CREATE)) {
 			$user_id = UserHelper::find_user_by_login($login);
@@ -42,7 +47,12 @@ abstract class Auth_Base extends Plugin implements IAuthModule {
 		return UserHelper::find_user_by_login($login);
 	}
 
-	// @deprecated
+
+	/** replaced with UserHelper::find_user_by_login()
+	 * @param string $login
+	 * @return null|int
+	 * @deprecated
+	 */
 	function find_user_by_login(string $login) {
 		return UserHelper::find_user_by_login($login);
 	}
diff --git a/classes/config.php b/classes/config.php
index 53a31b61c..6832846a2 100644
--- a/classes/config.php
+++ b/classes/config.php
@@ -231,9 +231,13 @@ class Config {
 																					Config::T_STRING ],
 	];
 
+	/** @var Config|null */
 	private static $instance;
 
+	/** @var array<string, array<bool|int|string>> */
 	private $params = [];
+
+	/** @var array<string, mixed> */
 	private $version = [];
 
 	/** @var Db_Migrations|null $migrations */
@@ -268,10 +272,16 @@ class Config {
 		directory, its contents are displayed instead of git commit-based version, this could be generated
 		based on source git tree commit used when creating the package */
 
+	/**
+	 * @return array<string, mixed>|string
+	 */
 	static function get_version(bool $as_string = true) {
 		return self::get_instance()->_get_version($as_string);
 	}
 
+	/**
+	 * @return array<string, mixed>|string
+	 */
 	private function _get_version(bool $as_string = true) {
 		$root_dir = dirname(__DIR__);
 
@@ -289,7 +299,10 @@ class Config {
 					user_error("Unable to determine version: " . $this->version["version"], E_USER_WARNING);
 
 					$this->version["version"] = "UNKNOWN (Unsupported, Git error)";
+				} else if (!getenv("TTRSS_SELF_URL_PATH") || !file_exists("/.dockerenv")) {
+					$this->version["version"] .= " (Unsupported)";
 				}
+
 			} else {
 				$this->version["version"] = "UNKNOWN (Unsupported)";
 			}
@@ -298,7 +311,10 @@ class Config {
 		return $as_string ? $this->version["version"] : $this->version;
 	}
 
-	static function get_version_from_git(string $dir) {
+	/**
+	 * @return array<string, int|string>
+	 */
+	static function get_version_from_git(string $dir): array {
 		$descriptorspec = [
 			1 => ["pipe", "w"], // STDOUT
 			2 => ["pipe", "w"], // STDERR
@@ -364,6 +380,9 @@ class Config {
 		return self::get_migrations()->get_version();
 	}
 
+	/**
+	 * @return bool|int|string
+	 */
 	static function cast_to(string $value, int $type_hint) {
 		switch ($type_hint) {
 			case self::T_BOOL:
@@ -375,24 +394,30 @@ class Config {
 		}
 	}
 
+	/**
+	 * @return bool|int|string
+	 */
 	private function _get(string $param) {
 		list ($value, $type_hint) = $this->params[$param];
 
 		return $this->cast_to($value, $type_hint);
 	}
 
-	private function _add(string $param, string $default, int $type_hint) {
+	private function _add(string $param, string $default, int $type_hint): void {
 		$override = getenv(self::_ENVVAR_PREFIX . $param);
 
 		$this->params[$param] = [ self::cast_to($override !== false ? $override : $default, $type_hint), $type_hint ];
 	}
 
-	static function add(string $param, string $default, int $type_hint = Config::T_STRING) {
+	static function add(string $param, string $default, int $type_hint = Config::T_STRING): void {
 		$instance = self::get_instance();
 
-		return $instance->_add($param, $default, $type_hint);
+		$instance->_add($param, $default, $type_hint);
 	}
 
+	/**
+	 * @return bool|int|string
+	 */
 	static function get(string $param) {
 		$instance = self::get_instance();
 
@@ -431,6 +456,9 @@ class Config {
 
 	/* sanity check stuff */
 
+	/**
+	 * @return array<int, array<string, string>> A list of entries identifying tt-rss tables with bad config
+	 */
 	private static function check_mysql_tables() {
 		$pdo = Db::pdo();
 
@@ -447,7 +475,7 @@ class Config {
 		return $bad_tables;
 	}
 
-	static function sanity_check() {
+	static function sanity_check(): void {
 
 		/*
 			we don't actually need the DB object right now but some checks below might use ORM which won't be initialized
@@ -621,11 +649,11 @@ class Config {
 		}
 	}
 
-	private static function format_error($msg) {
+	private static function format_error(string $msg): string {
 		return "<div class=\"alert alert-danger\">$msg</div>";
 	}
 
-	static function get_override_links() {
+	static function get_override_links(): string {
 		$rv = "";
 
 		$local_css = get_theme_path(self::get(self::LOCAL_OVERRIDE_STYLESHEET));
@@ -637,7 +665,7 @@ class Config {
 		return $rv;
 	}
 
-	static function get_user_agent() {
+	static function get_user_agent(): string {
 		return sprintf(self::get(self::HTTP_USER_AGENT), self::get_version());
 	}
 }
diff --git a/classes/counters.php b/classes/counters.php
index 8a8b8bc71..c42e938f8 100644
--- a/classes/counters.php
+++ b/classes/counters.php
@@ -1,7 +1,10 @@
 <?php
 class Counters {
 
-	static function get_all() {
+	/**
+	 * @return array<int, array<string, int|string>>
+	 */
+	static function get_all(): array {
 		return array_merge(
 			self::get_global(),
 			self::get_virt(),
@@ -11,7 +14,12 @@ class Counters {
 		);
 	}
 
-	static function get_conditional(array $feed_ids = null, array $label_ids = null) {
+	/**
+	 * @param array<int> $feed_ids
+	 * @param array<int> $label_ids
+	 * @return array<int, array<string, int|string>>
+	 */
+	static function get_conditional(array $feed_ids = null, array $label_ids = null): array {
 		return array_merge(
 			self::get_global(),
 			self::get_virt(),
@@ -21,7 +29,10 @@ class Counters {
 		);
 	}
 
-	static private function get_cat_children(int $cat_id, int $owner_uid) {
+	/**
+	 * @return array<int, int>
+	 */
+	static private function get_cat_children(int $cat_id, int $owner_uid): array {
 		$unread = 0;
 		$marked = 0;
 
@@ -40,7 +51,11 @@ class Counters {
 		return [$unread, $marked];
 	}
 
-	private static function get_cats(array $cat_ids = null) {
+	/**
+	 * @param array<int> $cat_ids
+	 * @return array<int, array<string, int|string>>
+	 */
+	private static function get_cats(array $cat_ids = null): array {
 		$ret = [];
 
 		/* Labels category */
@@ -129,7 +144,11 @@ class Counters {
 		return $ret;
 	}
 
-	private static function get_feeds(array $feed_ids = null) {
+	/**
+	 * @param array<int> $feed_ids
+	 * @return array<int, array<string, int|string>>
+	 */
+	private static function get_feeds(array $feed_ids = null): array {
 
 		$ret = [];
 
@@ -199,7 +218,10 @@ class Counters {
 		return $ret;
 	}
 
-	private static function get_global() {
+	/**
+	 * @return array<int, array<string, int|string>>
+	 */
+	private static function get_global(): array {
 		$ret = [
 			[
 				"id" => "global-unread",
@@ -219,7 +241,10 @@ class Counters {
 		return $ret;
 	}
 
-	private static function get_virt() {
+	/**
+	 * @return array<int, array<string, int|string>>
+	 */
+	private static function get_virt(): array {
 
 		$ret = [];
 
@@ -248,6 +273,11 @@ class Counters {
 
 		if (is_array($feeds)) {
 			foreach ($feeds as $feed) {
+				/** @var IVirtualFeed $feed['sender'] */
+
+				if (!implements_interface($feed['sender'], 'IVirtualFeed'))
+					continue;
+
 				$cv = [
 					"id" => PluginHost::pfeed_to_feed_id($feed['id']),
 					"counter" => $feed['sender']->get_unread($feed['id'])
@@ -263,7 +293,11 @@ class Counters {
 		return $ret;
 	}
 
-	static function get_labels(array $label_ids = null) {
+	/**
+	 * @param array<int> $label_ids
+	 * @return array<int, array<string, int|string>>
+	 */
+	static function get_labels(array $label_ids = null): array {
 
 		$ret = [];
 
diff --git a/classes/db.php b/classes/db.php
index 7b669cf32..2cc89f5ba 100755
--- a/classes/db.php
+++ b/classes/db.php
@@ -17,7 +17,7 @@ class Db
 		}
 	}
 
-	static function NOW() {
+	static function NOW(): string {
 		return date("Y-m-d H:i:s", time());
 	}
 
@@ -25,7 +25,7 @@ class Db
 		//
 	}
 
-	public static function get_dsn() {
+	public static function get_dsn(): string {
 		$db_port = Config::get(Config::DB_PORT) ? ';port=' . Config::get(Config::DB_PORT) : '';
 		$db_host = Config::get(Config::DB_HOST) ? ';host=' . Config::get(Config::DB_HOST) : '';
 		if (Config::get(Config::DB_TYPE) == "mysql" && Config::get(Config::MYSQL_CHARSET)) {
@@ -88,12 +88,11 @@ class Db
 		return self::$instance->pdo;
 	}
 
-	public static function sql_random_function() {
+	public static function sql_random_function(): string {
 		if (Config::get(Config::DB_TYPE) == "mysql") {
 			return "RAND()";
-		} else {
-			return "RANDOM()";
 		}
+		return "RANDOM()";
 	}
 
 }
diff --git a/classes/db/migrations.php b/classes/db/migrations.php
index 3008af535..aecd9186c 100644
--- a/classes/db/migrations.php
+++ b/classes/db/migrations.php
@@ -1,29 +1,46 @@
 <?php
 class Db_Migrations {
 
+	// TODO: class properties can be switched to PHP typing if/when the minimum PHP_VERSION is raised to 7.4.0+
+	/** @var string */
 	private $base_filename = "schema.sql";
+
+	/** @var string */
 	private $base_path;
+
+	/** @var string */
 	private $migrations_path;
+
+	/** @var string */
 	private $migrations_table;
+
+	/** @var bool */
 	private $base_is_latest;
+
+	/** @var PDO */
 	private $pdo;
 
-	private $cached_version;
-	private $cached_max_version;
+	/** @var int */
+	private $cached_version = 0;
+
+	/** @var int */
+	private $cached_max_version = 0;
+
+	/** @var int */
 	private $max_version_override;
 
 	function __construct() {
 		$this->pdo = Db::pdo();
 	}
 
-	function initialize_for_plugin(Plugin $plugin, bool $base_is_latest = true, string $schema_suffix = "sql") {
+	function initialize_for_plugin(Plugin $plugin, bool $base_is_latest = true, string $schema_suffix = "sql"): void {
 		$plugin_dir = PluginHost::getInstance()->get_plugin_dir($plugin);
 		$this->initialize($plugin_dir . "/${schema_suffix}",
 			strtolower("ttrss_migrations_plugin_" . get_class($plugin)),
 			$base_is_latest);
 	}
 
-	function initialize(string $root_path, string $migrations_table, bool $base_is_latest = true, int $max_version_override = 0) {
+	function initialize(string $root_path, string $migrations_table, bool $base_is_latest = true, int $max_version_override = 0): void {
 		$this->base_path = "$root_path/" . Config::get(Config::DB_TYPE);
 		$this->migrations_path = $this->base_path . "/migrations";
 		$this->migrations_table = $migrations_table;
@@ -31,7 +48,7 @@ class Db_Migrations {
 		$this->max_version_override =  $max_version_override;
 	}
 
-	private function set_version(int $version) {
+	private function set_version(int $version): void {
 		Debug::log("Updating table {$this->migrations_table} with version ${version}...", Debug::LOG_EXTENDED);
 
 		$sth = $this->pdo->query("SELECT * FROM {$this->migrations_table}");
@@ -48,7 +65,7 @@ class Db_Migrations {
 	}
 
 	function get_version() : int {
-		if (isset($this->cached_version))
+		if ($this->cached_version)
 			return $this->cached_version;
 
 		try {
@@ -66,11 +83,15 @@ class Db_Migrations {
 		}
 	}
 
-	private function create_migrations_table() {
+	private function create_migrations_table(): void {
 		$this->pdo->query("CREATE TABLE IF NOT EXISTS {$this->migrations_table} (schema_version integer not null)");
 	}
 
-	private function migrate_to(int $version) {
+	/**
+	 * @throws PDOException
+	 * @return bool false if the migration failed, otherwise true (or an exception)
+	 */
+	private function migrate_to(int $version): bool {
 		try {
 			if ($version <= $this->get_version()) {
 				Debug::log("Refusing to apply version $version: current version is higher", Debug::LOG_VERBOSE);
@@ -110,8 +131,10 @@ class Db_Migrations {
 				Debug::log("Migration finished, current version: " . $this->get_version(), Debug::LOG_VERBOSE);
 
 				Logger::log(E_USER_NOTICE, "Applied migration to version $version for {$this->migrations_table}");
+				return true;
 			} else {
 				Debug::log("Migration failed: schema file is empty or missing.", Debug::LOG_VERBOSE);
+				return false;
 			}
 
 		} catch (PDOException $e) {
@@ -129,7 +152,7 @@ class Db_Migrations {
 		if ($this->max_version_override > 0)
 			return $this->max_version_override;
 
-		if (isset($this->cached_max_version))
+		if ($this->cached_max_version)
 			return $this->cached_max_version;
 
 		$migrations = glob("{$this->migrations_path}/*.sql");
@@ -174,6 +197,9 @@ class Db_Migrations {
 		return !$this->is_migration_needed();
 	}
 
+	/**
+	 * @return array<int, string>
+	 */
 	private function get_lines(int $version) : array {
 		if ($version > 0)
 			$filename = "{$this->migrations_path}/${version}.sql";
diff --git a/classes/db/prefs.php b/classes/db/prefs.php
index 821216622..209ef58c1 100644
--- a/classes/db/prefs.php
+++ b/classes/db/prefs.php
@@ -2,11 +2,17 @@
 class Db_Prefs {
 	// this class is a stub for the time being (to be removed)
 
-	function read($pref_name, $user_id = false, $die_on_error = false) {
+	/**
+	 * @return bool|int|null|string
+	 */
+	function read(string $pref_name, ?int $user_id = null, bool $die_on_error = false) {
 		return get_pref($pref_name, $user_id);
 	}
 
-	function write($pref_name, $value, $user_id = false, $strip_tags = true) {
+	/**
+	 * @param mixed $value
+	 */
+	function write(string $pref_name, $value, ?int $user_id = null, bool $strip_tags = true): bool {
 		return set_pref($pref_name, $value, $user_id, $strip_tags);
 	}
 }
diff --git a/classes/debug.php b/classes/debug.php
index 2ae81e41a..e20126b86 100644
--- a/classes/debug.php
+++ b/classes/debug.php
@@ -1,52 +1,90 @@
 <?php
 class Debug {
 	const LOG_DISABLED = -1;
-    const LOG_NORMAL = 0;
-    const LOG_VERBOSE = 1;
-    const LOG_EXTENDED = 2;
-
-	/** @deprecated */
+	const LOG_NORMAL = 0;
+	const LOG_VERBOSE = 1;
+	const LOG_EXTENDED = 2;
+
+	const ALL_LOG_LEVELS = [
+		Debug::LOG_DISABLED,
+		Debug::LOG_NORMAL,
+		Debug::LOG_VERBOSE,
+		Debug::LOG_EXTENDED,
+	];
+
+	// TODO: class properties can be switched to PHP typing if/when the minimum PHP_VERSION is raised to 7.4.0+
+	/**
+	 * @deprecated
+	 * @var int
+	*/
 	public static $LOG_DISABLED = self::LOG_DISABLED;
 
-	/** @deprecated */
-    public static $LOG_NORMAL = self::LOG_NORMAL;
+	/**
+	 * @deprecated
+	 * @var int
+	*/
+	public static $LOG_NORMAL = self::LOG_NORMAL;
+
+	/**
+	 * @deprecated
+	 * @var int
+	*/
+	public static $LOG_VERBOSE = self::LOG_VERBOSE;
+
+	/**
+	 * @deprecated
+	 * @var int
+	*/
+	public static $LOG_EXTENDED = self::LOG_EXTENDED;
 
-	/** @deprecated */
-    public static $LOG_VERBOSE = self::LOG_VERBOSE;
+	/** @var bool */
+	private static $enabled = false;
 
-	/** @deprecated */
-    public static $LOG_EXTENDED = self::LOG_EXTENDED;
+	/** @var bool */
+	private static $quiet = false;
 
-    private static $enabled = false;
-    private static $quiet = false;
-    private static $logfile = false;
+	/** @var string|null */
+	private static $logfile = null;
+
+	/**
+	 * @var int Debug::LOG_*
+	 */
     private static $loglevel = self::LOG_NORMAL;
 
-	public static function set_logfile($logfile) {
+	public static function set_logfile(string $logfile): void {
         self::$logfile = $logfile;
     }
 
-    public static function enabled() {
+    public static function enabled(): bool {
         return self::$enabled;
     }
 
-    public static function set_enabled($enable) {
+    public static function set_enabled(bool $enable): void {
         self::$enabled = $enable;
     }
 
-    public static function set_quiet($quiet) {
+    public static function set_quiet(bool $quiet): void {
         self::$quiet = $quiet;
     }
 
-    public static function set_loglevel($level) {
+	/**
+	 * @param int $level Debug::LOG_*
+	 */
+    public static function set_loglevel(int $level): void {
         self::$loglevel = $level;
     }
 
-    public static function get_loglevel() {
+	/**
+	 * @return int Debug::LOG_*
+	 */
+    public static function get_loglevel(): int {
         return self::$loglevel;
     }
 
-    public static function log($message, int $level = 0) {
+	/**
+	 * @param int $level Debug::LOG_*
+	 */
+    public static function log(string $message, int $level = Debug::LOG_NORMAL): bool {
 
         if (!self::$enabled || self::$loglevel < $level) return false;
 
@@ -73,7 +111,7 @@ class Debug {
                     if (!$locked) {
                         fclose($fp);
                         user_error("Unable to lock debugging log file: " . self::$logfile, E_USER_WARNING);
-                        return;
+                        return false;
                     }
                 }
 
@@ -86,7 +124,7 @@ class Debug {
                 fclose($fp);
 
                 if (self::$quiet)
-                    return;
+                    return false;
 
             } else {
                 user_error("Unable to open debugging log file: " . self::$logfile, E_USER_WARNING);
@@ -94,5 +132,7 @@ class Debug {
         }
 
         print "[$ts] $message\n";
+
+		return true;
     }
 }
diff --git a/classes/digest.php b/classes/digest.php
index 94e5cd1fc..15203166b 100644
--- a/classes/digest.php
+++ b/classes/digest.php
@@ -1,7 +1,7 @@
 <?php
 class Digest
 {
-	static function send_headlines_digests() {
+	static function send_headlines_digests(): void {
 
 		$user_limit = 15; // amount of users to process (e.g. emails to send out)
 		$limit = 1000; // maximum amount of headlines to include
@@ -62,7 +62,7 @@ class Digest
 
 						if ($rc && $do_catchup) {
 							Debug::log("Marking affected articles as read...");
-							Article::_catchup_by_id($affected_ids, 0, $line["id"]);
+							Article::_catchup_by_id($affected_ids, Article::CATCHUP_MODE_MARK_AS_READ, $line["id"]);
 						}
 					} else {
 						Debug::log("No headlines");
@@ -78,6 +78,9 @@ class Digest
 		Debug::log("All done.");
 	}
 
+	/**
+	 * @return array{0: string, 1: int, 2: array<int>, 3: string}
+	 */
 	static function prepare_headlines_digest(int $user_id, int $days = 1, int $limit = 1000) {
 
 		$tpl = new Templator();
diff --git a/classes/diskcache.php b/classes/diskcache.php
index d7ea26d3b..0df8d7cd4 100644
--- a/classes/diskcache.php
+++ b/classes/diskcache.php
@@ -1,9 +1,15 @@
 <?php
 class DiskCache {
+	// TODO: class properties can be switched to PHP typing if/when the minimum PHP_VERSION is raised to 7.4.0+
+	/** @var string */
 	private $dir;
 
-	// https://stackoverflow.com/a/53662733
-	private $mimeMap = [
+	/**
+	 * https://stackoverflow.com/a/53662733
+	 *
+	 * @var array<string, string>
+	 */
+	private array $mimeMap = [
 		'video/3gpp2'                                                               => '3g2',
 		'video/3gp'                                                                 => '3gp',
 		'video/3gpp'                                                                => '3gp',
@@ -190,21 +196,22 @@ class DiskCache {
 		'text/x-scriptzsh'                                                          => 'zsh'
 	];
 
-	public function __construct($dir) {
+	public function __construct(string $dir) {
 		$this->dir = Config::get(Config::CACHE_DIR) . "/" . basename(clean($dir));
 	}
 
-	public function get_dir() {
+	public function get_dir(): string {
 		return $this->dir;
 	}
 
-	public function make_dir() {
+	public function make_dir(): bool {
 		if (!is_dir($this->dir)) {
 			return mkdir($this->dir);
 		}
+		return false;
 	}
 
-	public function is_writable($filename = "") {
+	public function is_writable(?string $filename = null): bool {
 		if ($filename) {
 			if (file_exists($this->get_full_path($filename)))
 				return is_writable($this->get_full_path($filename));
@@ -215,44 +222,75 @@ class DiskCache {
 		}
 	}
 
-	public function exists($filename) {
+	public function exists(string $filename): bool {
 		return file_exists($this->get_full_path($filename));
 	}
 
-	public function get_size($filename) {
+	/**
+	 * @return int|false -1 if the file doesn't exist, false if an error occurred, size in bytes otherwise
+	 */
+	public function get_size(string $filename) {
 		if ($this->exists($filename))
 			return filesize($this->get_full_path($filename));
 		else
 			return -1;
 	}
 
-	public function get_full_path($filename) {
+	public function get_full_path(string $filename): string {
 		return $this->dir . "/" . basename(clean($filename));
 	}
 
-	public function put($filename, $data) {
+	/**
+	 * @param mixed $data
+	 *
+	 * @return int|false Bytes written or false if an error occurred.
+	 */
+	public function put(string $filename, $data) {
 		return file_put_contents($this->get_full_path($filename), $data);
 	}
 
-	public function touch($filename) {
+	public function touch(string $filename): bool {
 		return touch($this->get_full_path($filename));
 	}
 
-	public function get($filename) {
+	/** Downloads $url to cache as $local_filename if its missing (unless $force-ed)
+	 * @param string $url
+	 * @param string $local_filename
+	 * @param array<string,string|int|false> $options (additional params to UrlHelper::fetch())
+	 * @param bool $force
+	 * @return bool
+	 */
+	public function download(string $url, string $local_filename, array $options = [], bool $force = false) : bool {
+		if ($this->exists($local_filename) && !$force)
+			return true;
+
+		$data = UrlHelper::fetch(array_merge(["url" => $url,
+							"max_size" => Config::get(Config::MAX_CACHE_FILE_SIZE)], $options));
+
+		if ($data)
+			return $this->put($local_filename, $data) > 0;
+
+		return false;
+	}
+
+	public function get(string $filename): ?string {
 		if ($this->exists($filename))
 			return file_get_contents($this->get_full_path($filename));
 		else
 			return null;
 	}
 
-	public function get_mime_type($filename) {
+	/**
+	 * @return false|null|string false if detection failed, null if the file doesn't exist, string mime content type otherwise
+	 */
+	public function get_mime_type(string $filename) {
 		if ($this->exists($filename))
 			return mime_content_type($this->get_full_path($filename));
 		else
 			return null;
 	}
 
-	public function get_fake_extension($filename) {
+	public function get_fake_extension(string $filename): string {
 		$mimetype = $this->get_mime_type($filename);
 
 		if ($mimetype)
@@ -261,7 +299,10 @@ class DiskCache {
 			return "";
 	}
 
-	public function send($filename) {
+	/**
+	 * @return bool|int false if the file doesn't exist (or unreadable) or isn't audio/video, true if a plugin handled, otherwise int of bytes sent
+	 */
+	public function send(string $filename) {
 		$fake_extension = $this->get_fake_extension($filename);
 
 		if ($fake_extension)
@@ -272,7 +313,7 @@ class DiskCache {
 		return $this->send_local_file($this->get_full_path($filename));
 	}
 
-	public function get_url($filename) {
+	public function get_url(string $filename): string {
 		return Config::get_self_url() . "/public.php?op=cached&file=" . basename($this->dir) . "/" . basename($filename);
 	}
 
@@ -280,8 +321,7 @@ class DiskCache {
 	// this is called separately after sanitize() and plugin render article hooks to allow
 	// plugins work on original source URLs used before caching
 	// NOTE: URLs should be already absolutized because this is called after sanitize()
-	static public function rewrite_urls($str)
-	{
+	static public function rewrite_urls(string $str): string {
 		$res = trim($str);
 		if (!$res) return '';
 
@@ -338,7 +378,7 @@ class DiskCache {
 		return $res;
 	}
 
-	static function expire() {
+	static function expire(): void {
 		$dirs = array_filter(glob(Config::get(Config::CACHE_DIR) . "/*"), "is_dir");
 
 		foreach ($dirs as $cache_dir) {
@@ -362,14 +402,19 @@ class DiskCache {
 		}
 	}
 
-	/*	this is essentially a wrapper for readfile() which allows plugins to hook
-		output with httpd-specific "fast" implementation i.e. X-Sendfile or whatever else
-
-		hook function should return true if request was handled (or at least attempted to)
-
-		note that this can be called without user context so the plugin to handle this
-		should be loaded systemwide in config.php */
-	function send_local_file($filename) {
+	/*	 */
+	/**
+	 * this is essentially a wrapper for readfile() which allows plugins to hook
+	 * output with httpd-specific "fast" implementation i.e. X-Sendfile or whatever else
+	 *
+	 * hook function should return true if request was handled (or at least attempted to)
+	 *
+	 * note that this can be called without user context so the plugin to handle this
+	 * should be loaded systemwide in config.php
+	 *
+	 * @return bool|int false if the file doesn't exist (or unreadable) or isn't audio/video, true if a plugin handled, otherwise int of bytes sent
+	 */
+	function send_local_file(string $filename) {
 		if (file_exists($filename)) {
 
 			if (is_writable($filename)) touch($filename);
diff --git a/classes/errors.php b/classes/errors.php
index 3599c2639..31be558cf 100644
--- a/classes/errors.php
+++ b/classes/errors.php
@@ -7,7 +7,11 @@ class Errors {
 	const E_SCHEMA_MISMATCH = "E_SCHEMA_MISMATCH";
 	const E_URL_SCHEME_MISMATCH = "E_URL_SCHEME_MISMATCH";
 
-	static function to_json(string $code, array $params = []) {
+	/**
+	 * @param Errors::E_* $code
+	 * @param array<string, string> $params
+	 */
+	static function to_json(string $code, array $params = []): string {
 		return json_encode(["error" => ["code" => $code, "params" => $params]]);
 	}
 }
diff --git a/classes/feedenclosure.php b/classes/feedenclosure.php
index 2435f6854..b5f5cc411 100644
--- a/classes/feedenclosure.php
+++ b/classes/feedenclosure.php
@@ -1,10 +1,21 @@
 <?php
 class FeedEnclosure {
+	/** @var string */
 	public $link;
+
+	/** @var string */
 	public $type;
+
+	/** @var string */
 	public $length;
+
+	/** @var string */
 	public $title;
+
+	/** @var string */
 	public $height;
+
+	/** @var string */
 	public $width;
 }
 
diff --git a/classes/feeditem.php b/classes/feeditem.php
index 3a5e5dc09..fd7c54883 100644
--- a/classes/feeditem.php
+++ b/classes/feeditem.php
@@ -1,16 +1,24 @@
 <?php
 abstract class FeedItem {
-	abstract function get_id();
+	abstract function get_id(): string;
+
+	/** @return int|false a timestamp on success, false otherwise */
 	abstract function get_date();
-	abstract function get_link();
-	abstract function get_title();
-	abstract function get_description();
-	abstract function get_content();
-	abstract function get_comments_url();
-	abstract function get_comments_count();
-	abstract function get_categories();
-	abstract function get_enclosures();
-	abstract function get_author();
-	abstract function get_language();
+
+	abstract function get_link(): string;
+	abstract function get_title(): string;
+	abstract function get_description(): string;
+	abstract function get_content(): string;
+	abstract function get_comments_url(): string;
+	abstract function get_comments_count(): int;
+
+	/** @return array<int, string> */
+	abstract function get_categories(): array;
+
+	/** @return array<int, FeedEnclosure> */
+	abstract function get_enclosures(): array;
+
+	abstract function get_author(): string;
+	abstract function get_language(): string;
 }
 
diff --git a/classes/feeditem/atom.php b/classes/feeditem/atom.php
index 51358f36c..36a2e91f5 100755
--- a/classes/feeditem/atom.php
+++ b/classes/feeditem/atom.php
@@ -2,7 +2,7 @@
 class FeedItem_Atom extends FeedItem_Common {
 	const NS_XML = "http://www.w3.org/XML/1998/namespace";
 
-	function get_id() {
+	function get_id(): string {
 		$id = $this->elem->getElementsByTagName("id")->item(0);
 
 		if ($id) {
@@ -12,6 +12,9 @@ class FeedItem_Atom extends FeedItem_Common {
 		}
 	}
 
+	/**
+	 * @return int|false a timestamp on success, false otherwise
+	 */
 	function get_date() {
 		$updated = $this->elem->getElementsByTagName("updated")->item(0);
 
@@ -30,10 +33,13 @@ class FeedItem_Atom extends FeedItem_Common {
 		if ($date) {
 			return strtotime($date->nodeValue);
 		}
+
+		// consistent with strtotime failing to parse
+		return false;
 	}
 
 
-	function get_link() {
+	function get_link(): string {
 		$links = $this->elem->getElementsByTagName("link");
 
 		foreach ($links as $link) {
@@ -44,24 +50,27 @@ class FeedItem_Atom extends FeedItem_Common {
 				$base = $this->xpath->evaluate("string(ancestor-or-self::*[@xml:base][1]/@xml:base)", $link);
 
 				if ($base)
-					return rewrite_relative_url($base, clean(trim($link->getAttribute("href"))));
+					return UrlHelper::rewrite_relative($base, clean(trim($link->getAttribute("href"))));
 				else
 					return clean(trim($link->getAttribute("href")));
-
 			}
 		}
+
+		return '';
 	}
 
-	function get_title() {
+	function get_title(): string {
 		$title = $this->elem->getElementsByTagName("title")->item(0);
-
-		if ($title) {
-			return clean(trim($title->nodeValue));
-		}
+		return $title ? clean(trim($title->nodeValue)) : '';
 	}
 
-	/** $base is optional (returns $content if $base is null), $content is an HTML string */
-	private function rewrite_content_to_base($base, $content) {
+	/**
+	 * @param string|null $base optional (returns $content if $base is null)
+	 * @param string $content an HTML string
+	 *
+	 * @return string the rewritten XML or original $content
+	 */
+	private function rewrite_content_to_base(?string $base = null, string $content) {
 
 		if (!empty($base) && !empty($content)) {
 
@@ -81,14 +90,17 @@ class FeedItem_Atom extends FeedItem_Common {
 					}
 				}
 
-				return $tmpdoc->saveXML();
+				// Fall back to $content if saveXML somehow fails (i.e. returns false)
+				$modified_content = $tmpdoc->saveXML();
+				return $modified_content !== false ? $modified_content : $content;
 			}
 		}
 
 		return $content;
 	}
 
-	function get_content() {
+	function get_content(): string {
+		/** @var DOMElement|null */
 		$content = $this->elem->getElementsByTagName("content")->item(0);
 
 		if ($content) {
@@ -108,10 +120,13 @@ class FeedItem_Atom extends FeedItem_Common {
 
 			return $this->rewrite_content_to_base($base, $this->subtree_or_text($content));
 		}
+
+		return '';
 	}
 
 	// TODO: duplicate code should be merged with get_content()
-	function get_description() {
+	function get_description(): string {
+		/** @var DOMElement|null */
 		$content = $this->elem->getElementsByTagName("summary")->item(0);
 
 		if ($content) {
@@ -132,9 +147,13 @@ class FeedItem_Atom extends FeedItem_Common {
 			return $this->rewrite_content_to_base($base, $this->subtree_or_text($content));
 		}
 
+		return '';
 	}
 
-	function get_categories() {
+	/**
+	 * @return array<int, string>
+	 */
+	function get_categories(): array {
 		$categories = $this->elem->getElementsByTagName("category");
 		$cats = [];
 
@@ -152,7 +171,10 @@ class FeedItem_Atom extends FeedItem_Common {
 		return $this->normalize_categories($cats);
 	}
 
-	function get_enclosures() {
+	/**
+	 * @return array<int, FeedEnclosure>
+	 */
+	function get_enclosures(): array {
 		$links = $this->elem->getElementsByTagName("link");
 
 		$encs = [];
@@ -182,7 +204,7 @@ class FeedItem_Atom extends FeedItem_Common {
 		return $encs;
 	}
 
-	function get_language() {
+	function get_language(): string {
 		$lang = $this->elem->getAttributeNS(self::NS_XML, "lang");
 
 		if (!empty($lang)) {
@@ -195,5 +217,6 @@ class FeedItem_Atom extends FeedItem_Common {
 				}
 			}
 		}
+		return '';
 	}
 }
diff --git a/classes/feeditem/common.php b/classes/feeditem/common.php
index 18afeaa94..6a9be8aca 100755
--- a/classes/feeditem/common.php
+++ b/classes/feeditem/common.php
@@ -1,16 +1,20 @@
 <?php
 abstract class FeedItem_Common extends FeedItem {
+	/** @var DOMElement */
 	protected $elem;
-	protected $xpath;
+
+	/** @var DOMDocument */
 	protected $doc;
 
-	function __construct($elem, $doc, $xpath) {
+	/** @var DOMXPath */
+	protected $xpath;
+
+	function __construct(DOMElement $elem, DOMDocument $doc, DOMXPath $xpath) {
 		$this->elem = $elem;
 		$this->xpath = $xpath;
 		$this->doc = $doc;
 
 		try {
-
 			$source = $elem->getElementsByTagName("source")->item(0);
 
 			// we don't need <source> element
@@ -21,11 +25,12 @@ abstract class FeedItem_Common extends FeedItem {
 		}
 	}
 
-	function get_element() {
+	function get_element(): DOMElement {
 		return $this->elem;
 	}
 
-	function get_author() {
+	function get_author(): string {
+		/** @var DOMElement|null */
 		$author = $this->elem->getElementsByTagName("author")->item(0);
 
 		if ($author) {
@@ -51,7 +56,7 @@ abstract class FeedItem_Common extends FeedItem {
 		return implode(", ", $authors);
 	}
 
-	function get_comments_url() {
+	function get_comments_url(): string {
 		//RSS only. Use a query here to avoid namespace clashes (e.g. with slash).
 		//might give a wrong result if a default namespace was declared (possible with XPath 2.0)
 		$com_url = $this->xpath->query("comments", $this->elem)->item(0);
@@ -65,20 +70,28 @@ abstract class FeedItem_Common extends FeedItem {
 
 		if ($com_url)
 			return clean($com_url->nodeValue);
+
+		return '';
 	}
 
-	function get_comments_count() {
+	function get_comments_count(): int {
 		//also query for ATE stuff here
 		$query = "slash:comments|thread:total|atom:link[@rel='replies']/@thread:count";
 		$comments = $this->xpath->query($query, $this->elem)->item(0);
 
-		if ($comments) {
-			return clean($comments->nodeValue);
+		if ($comments && is_numeric($comments->nodeValue)) {
+			return (int) clean($comments->nodeValue);
 		}
+
+		return 0;
 	}
 
-	// this is common for both Atom and RSS types and deals with various media: elements
-	function get_enclosures() {
+	/**
+	 * this is common for both Atom and RSS types and deals with various 'media:' elements
+	 *
+	 * @return array<int, FeedEnclosure>
+	 */
+	function get_enclosures(): array {
 		$encs = [];
 
 		$enclosures = $this->xpath->query("media:content", $this->elem);
@@ -108,6 +121,7 @@ abstract class FeedItem_Common extends FeedItem {
 		foreach ($enclosures as $enclosure) {
 			$enc = new FeedEnclosure();
 
+			/** @var DOMElement|null */
 			$content = $this->xpath->query("media:content", $enclosure)->item(0);
 
 			if ($content) {
@@ -150,11 +164,14 @@ abstract class FeedItem_Common extends FeedItem {
 		return $encs;
 	}
 
-	function count_children($node) {
+	function count_children(DOMElement $node): int {
 		return $node->getElementsByTagName("*")->length;
 	}
 
-	function subtree_or_text($node) {
+	/**
+	 * @return false|string false on failure, otherwise string contents
+	 */
+	function subtree_or_text(DOMElement $node) {
 		if ($this->count_children($node) == 0) {
 			return $node->nodeValue;
 		} else {
@@ -162,7 +179,12 @@ abstract class FeedItem_Common extends FeedItem {
 		}
 	}
 
-	static function normalize_categories($cats) {
+	/**
+	 * @param array<int, string> $cats
+	 *
+	 * @return array<int, string>
+	 */
+	static function normalize_categories(array $cats): array {
 
 		$tmp = [];
 
diff --git a/classes/feeditem/rss.php b/classes/feeditem/rss.php
index 1f7953c51..7017d04e9 100755
--- a/classes/feeditem/rss.php
+++ b/classes/feeditem/rss.php
@@ -1,6 +1,6 @@
 <?php
 class FeedItem_RSS extends FeedItem_Common {
-	function get_id() {
+	function get_id(): string {
 		$id = $this->elem->getElementsByTagName("guid")->item(0);
 
 		if ($id) {
@@ -10,6 +10,9 @@ class FeedItem_RSS extends FeedItem_Common {
 		}
 	}
 
+	/**
+	 * @return int|false a timestamp on success, false otherwise
+	 */
 	function get_date() {
 		$pubDate = $this->elem->getElementsByTagName("pubDate")->item(0);
 
@@ -22,9 +25,12 @@ class FeedItem_RSS extends FeedItem_Common {
 		if ($date) {
 			return strtotime($date->nodeValue);
 		}
+
+		// consistent with strtotime failing to parse
+		return false;
 	}
 
-	function get_link() {
+	function get_link(): string {
 		$links = $this->xpath->query("atom:link", $this->elem);
 
 		foreach ($links as $link) {
@@ -37,6 +43,7 @@ class FeedItem_RSS extends FeedItem_Common {
 			}
 		}
 
+		/** @var DOMElement|null */
 		$link = $this->elem->getElementsByTagName("guid")->item(0);
 
 		if ($link && $link->hasAttributes() && $link->getAttribute("isPermaLink") == "true") {
@@ -48,9 +55,11 @@ class FeedItem_RSS extends FeedItem_Common {
 		if ($link) {
 			return clean(trim($link->nodeValue));
 		}
+
+		return '';
 	}
 
-	function get_title() {
+	function get_title(): string {
 		$title = $this->xpath->query("title", $this->elem)->item(0);
 
 		if ($title) {
@@ -64,10 +73,15 @@ class FeedItem_RSS extends FeedItem_Common {
 		if ($title) {
 			return clean(trim($title->nodeValue));
 		}
+
+		return '';
 	}
 
-	function get_content() {
+	function get_content(): string {
+		/** @var DOMElement|null */
 		$contentA = $this->xpath->query("content:encoded", $this->elem)->item(0);
+
+		/** @var DOMElement|null */
 		$contentB = $this->elem->getElementsByTagName("description")->item(0);
 
 		if ($contentA && !$contentB) {
@@ -85,17 +99,24 @@ class FeedItem_RSS extends FeedItem_Common {
 
 			return mb_strlen($resultA) > mb_strlen($resultB) ? $resultA : $resultB;
 		}
+
+		return '';
 	}
 
-	function get_description() {
+	function get_description(): string {
 		$summary = $this->elem->getElementsByTagName("description")->item(0);
 
 		if ($summary) {
 			return $summary->nodeValue;
 		}
+
+		return '';
 	}
 
-	function get_categories() {
+	/**
+	 * @return array<int, string>
+	 */
+	function get_categories(): array {
 		$categories = $this->elem->getElementsByTagName("category");
 		$cats = [];
 
@@ -112,7 +133,10 @@ class FeedItem_RSS extends FeedItem_Common {
 		return $this->normalize_categories($cats);
 	}
 
-	function get_enclosures() {
+	/**
+	 * @return array<int, FeedEnclosure>
+	 */
+	function get_enclosures(): array {
 		$enclosures = $this->elem->getElementsByTagName("enclosure");
 
 		$encs = array();
@@ -134,7 +158,7 @@ class FeedItem_RSS extends FeedItem_Common {
 		return $encs;
 	}
 
-	function get_language() {
+	function get_language(): string {
 		$languages = $this->doc->getElementsByTagName('language');
 
 		if (count($languages) == 0) {
@@ -143,5 +167,4 @@ class FeedItem_RSS extends FeedItem_Common {
 
 		return clean($languages[0]->textContent);
 	}
-
 }
diff --git a/classes/feedparser.php b/classes/feedparser.php
index daba271fb..6ce69cc89 100644
--- a/classes/feedparser.php
+++ b/classes/feedparser.php
@@ -1,19 +1,35 @@
 <?php
 class FeedParser {
+
+	/** @var DOMDocument */
 	private $doc;
-	private $error;
-	private $libxml_errors = array();
-	private $items;
+
+	/** @var string|null */
+	private $error = null;
+
+	/** @var array<string> */
+	private $libxml_errors = [];
+
+	/** @var array<FeedItem> */
+	private $items = [];
+
+	/** @var string|null */
 	private $link;
+
+	/** @var string|null */
 	private $title;
+
+	/** @var FeedParser::FEED_*|null */
 	private $type;
+
+	/** @var DOMXPath|null */
 	private $xpath;
 
 	const FEED_RDF = 0;
 	const FEED_RSS = 1;
 	const FEED_ATOM = 2;
 
-	function __construct($data) {
+	function __construct(string $data) {
 		libxml_use_internal_errors(true);
 		libxml_clear_errors();
 		$this->doc = new DOMDocument();
@@ -26,18 +42,18 @@ class FeedParser {
 		if ($error) {
 			foreach (libxml_get_errors() as $error) {
 				if ($error->level == LIBXML_ERR_FATAL) {
-					if(!isset($this->error)) //currently only the first error is reported
+					// currently only the first error is reported
+					if (!isset($this->error)) {
 						$this->error = $this->format_error($error);
-					$this->libxml_errors [] = $this->format_error($error);
+					}
+					$this->libxml_errors[] = $this->format_error($error);
 				}
 			}
 		}
 		libxml_clear_errors();
-
-		$this->items = array();
 	}
 
-	function init() {
+	function init() : void {
 		$root = $this->doc->firstChild;
 		$xpath = new DOMXPath($this->doc);
 		$xpath->registerNamespace('atom', 'http://www.w3.org/2005/Atom');
@@ -51,10 +67,12 @@ class FeedParser {
 
 		$this->xpath = $xpath;
 
-		$root = $xpath->query("(//atom03:feed|//atom:feed|//channel|//rdf:rdf|//rdf:RDF)");
+		$root_list = $xpath->query("(//atom03:feed|//atom:feed|//channel|//rdf:rdf|//rdf:RDF)");
+
+		if (!empty($root_list) && $root_list->length > 0) {
 
-		if (!empty($root) && $root->length > 0) {
-			$root = $root->item(0);
+			/** @var DOMElement|null $root */
+			$root = $root_list->item(0);
 
 			if ($root) {
 				switch (mb_strtolower($root->tagName)) {
@@ -69,7 +87,7 @@ class FeedParser {
 					$this->type = $this::FEED_ATOM;
 					break;
 				default:
-					if( !isset($this->error) ){
+					if (!isset($this->error)) {
 						$this->error = "Unknown/unsupported feed type";
 					}
 					return;
@@ -100,6 +118,7 @@ class FeedParser {
 				if (!$link)
 					$link = $xpath->query("//atom03:feed/atom03:link[@rel='alternate']")->item(0);
 
+				/** @var DOMElement|null $link */
 				if ($link && $link->hasAttributes()) {
 					$this->link = $link->getAttribute("href");
 				}
@@ -121,6 +140,7 @@ class FeedParser {
 					$this->title = $title->nodeValue;
 				}
 
+				/** @var DOMElement|null $link */
 				$link = $xpath->query("//channel/link")->item(0);
 
 				if ($link) {
@@ -166,46 +186,44 @@ class FeedParser {
 			if ($this->link) $this->link = trim($this->link);
 
 		} else {
-			if( !isset($this->error) ){
+			if (!isset($this->error)) {
 				$this->error = "Unknown/unsupported feed type";
 			}
 			return;
 		}
 	}
 
-	function format_error($error) {
-		if ($error) {
-			return sprintf("LibXML error %s at line %d (column %d): %s",
-				$error->code, $error->line, $error->column,
-				$error->message);
-		} else {
-			return "";
-		}
+	function format_error(LibXMLError $error) : string {
+		return sprintf("LibXML error %s at line %d (column %d): %s",
+			$error->code, $error->line, $error->column,
+			$error->message);
 	}
 
 	// libxml may have invalid unicode data in error messages
-	function error() {
+	function error() : string {
 		return UConverter::transcode($this->error, 'UTF-8', 'UTF-8');
 	}
 
-	// WARNING: may return invalid unicode data
-	function errors() {
+	/** @return array<string> - WARNING: may return invalid unicode data */
+	function errors() : array {
 		return $this->libxml_errors;
 	}
 
-	function get_link() {
-		return clean($this->link);
+	function get_link() : string {
+		return clean($this->link ?? '');
 	}
 
-	function get_title() {
-		return clean($this->title);
+	function get_title() : string {
+		return clean($this->title ?? '');
 	}
 
-	function get_items() {
+	/** @return array<FeedItem> */
+	function get_items() : array {
 		return $this->items;
 	}
 
-	function get_links($rel) {
+	/** @return array<string> */
+	function get_links(string $rel) : array {
 		$rv = array();
 
 		switch ($this->type) {
diff --git a/classes/feeds.php b/classes/feeds.php
index cd2633ffb..0c75215c8 100755
--- a/classes/feeds.php
+++ b/classes/feeds.php
@@ -5,18 +5,24 @@ class Feeds extends Handler_Protected {
 	const NEVER_GROUP_FEEDS = [ -6, 0 ];
 	const NEVER_GROUP_BY_DATE = [ -2, -1, -3 ];
 
-	 private $viewfeed_timestamp;
-	 private $viewfeed_timestamp_last;
+	/** @var int|float int on 64-bit, float on 32-bit */
+	private $viewfeed_timestamp;
 
-    function csrf_ignore($method) {
+	/** @var int|float int on 64-bit, float on 32-bit */
+	private $viewfeed_timestamp_last;
+
+	function csrf_ignore(string $method): bool {
 		$csrf_ignored = array("index");
 
 		return array_search($method, $csrf_ignored) !== false;
 	}
 
-	private function _format_headlines_list($feed, $method, $view_mode, $limit, $cat_view,
-					$offset, $override_order = false, $include_children = false, $check_first_id = false,
-					$skip_first_id_check = false, $order_by = false) {
+	/**
+	 * @return array{0: array<int, int>, 1: int, 2: int, 3: bool, 4: array<string, mixed>} $topmost_article_ids, $headlines_count, $feed, $disable_cache, $reply
+	 */
+	private function _format_headlines_list(int $feed, string $method, string $view_mode, int $limit, bool $cat_view,
+					int $offset, string $override_order, bool $include_children, ?int $check_first_id = null,
+					bool $skip_first_id_check, string $order_by): array {
 
 		$disable_cache = false;
 
@@ -80,6 +86,8 @@ class Feeds extends Handler_Protected {
 					"include_children" => $include_children,
 					"order_by" => $order_by);
 
+				// Implemented by a plugin, so ignore the undefined 'get_headlines' method.
+				// @phpstan-ignore-next-line
 				$qfh_ret = $handler->get_headlines(PluginHost::feed_to_pfeed_id($feed),
 					$options);
 			}
@@ -271,7 +279,7 @@ class Feeds extends Handler_Protected {
 				$this->_mark_timestamp("   pre-sanitize");
 
 				$line["content"] = Sanitizer::sanitize($line["content"],
-					$line['hide_images'], false, $line["site_url"], $highlight_words, $line["id"]);
+					$line['hide_images'], null, $line["site_url"], $highlight_words, $line["id"]);
 
 				$this->_mark_timestamp("   sanitize");
 
@@ -289,9 +297,9 @@ class Feeds extends Handler_Protected {
 
 				if ($line["num_enclosures"] > 0) {
 					$line["enclosures"] = Article::_format_enclosures($id,
-						$line["always_display_enclosures"],
+						sql_bool_to_bool($line["always_display_enclosures"]),
 						$line["content"],
-						$line["hide_images"]);
+						sql_bool_to_bool($line["hide_images"]));
 				} else {
 					$line["enclosures"] = [ 'formatted' => '', 'entries' => [] ];
 				}
@@ -299,7 +307,7 @@ class Feeds extends Handler_Protected {
 				$this->_mark_timestamp("   enclosures");
 
 				$line["updated_long"] = TimeHelper::make_local_datetime($line["updated"],true);
-				$line["updated"] = TimeHelper::make_local_datetime($line["updated"], false, false, false, true);
+				$line["updated"] = TimeHelper::make_local_datetime($line["updated"], false, null, false, true);
 
 				$line['imported'] = T_sprintf("Imported at %s",
 					TimeHelper::make_local_datetime($line["date_entered"], false));
@@ -433,7 +441,7 @@ class Feeds extends Handler_Protected {
 		return array($topmost_article_ids, $headlines_count, $feed, $disable_cache, $reply);
 	}
 
-	function catchupAll() {
+	function catchupAll(): void {
 		$sth = $this->pdo->prepare("UPDATE ttrss_user_entries SET
 						last_read = NOW(), unread = false WHERE unread = true AND owner_uid = ?");
 		$sth->execute([$_SESSION['uid']]);
@@ -441,7 +449,7 @@ class Feeds extends Handler_Protected {
 		print json_encode(array("message" => "UPDATE_COUNTERS"));
 	}
 
-	function view() {
+	function view(): void {
 		$reply = array();
 
 		$feed = $_REQUEST["feed"];
@@ -450,7 +458,7 @@ class Feeds extends Handler_Protected {
 		$limit = 30;
 		$cat_view = $_REQUEST["cat"] == "true";
 		$next_unread_feed = $_REQUEST["nuf"] ?? 0;
-		$offset = $_REQUEST["skip"] ?? 0;
+		$offset = (int) ($_REQUEST["skip"] ?? 0);
 		$order_by = $_REQUEST["order_by"] ?? "";
 		$check_first_id = $_REQUEST["fid"] ?? 0;
 
@@ -538,7 +546,10 @@ class Feeds extends Handler_Protected {
 		print json_encode($reply);
 	}
 
-	private function _generate_dashboard_feed() {
+	/**
+	 * @return array<string, array<string, mixed>>
+	 */
+	private function _generate_dashboard_feed(): array {
 		$reply = array();
 
 		$reply['headlines']['id'] = -5;
@@ -580,7 +591,10 @@ class Feeds extends Handler_Protected {
 		return $reply;
 	}
 
-	private function _generate_error_feed($error) {
+	/**
+	 * @return array<string, mixed>
+	 */
+	private function _generate_error_feed(string $error): array {
 		$reply = array();
 
 		$reply['headlines']['id'] = -7;
@@ -596,13 +610,13 @@ class Feeds extends Handler_Protected {
 		return $reply;
 	}
 
-	function subscribeToFeed() {
+	function subscribeToFeed(): void {
 		print json_encode([
 			"cat_select" => \Controls\select_feeds_cats("cat")
 		]);
 	}
 
-	function search() {
+	function search(): void {
 		print json_encode([
 			"show_language" => Config::get(Config::DB_TYPE) == "pgsql",
 			"show_syntax_help" => count(PluginHost::getInstance()->get_hooks(PluginHost::HOOK_SEARCH)) == 0,
@@ -611,7 +625,7 @@ class Feeds extends Handler_Protected {
 		]);
 	}
 
-	function opensite() {
+	function opensite(): void {
 		$feed = ORM::for_table('ttrss_feeds')
 			->find_one((int)$_REQUEST['feed_id']);
 
@@ -628,10 +642,14 @@ class Feeds extends Handler_Protected {
 		print "Feed not found or has an empty site URL.";
 	}
 
-	function updatedebugger() {
+	function updatedebugger(): void {
 		header("Content-type: text/html");
 
-		$xdebug = isset($_REQUEST["xdebug"]) ? (int)$_REQUEST["xdebug"] : 1;
+		$xdebug = isset($_REQUEST["xdebug"]) ? (int)$_REQUEST["xdebug"] : Debug::LOG_VERBOSE;
+
+		if (!in_array($xdebug, Debug::ALL_LOG_LEVELS)) {
+			$xdebug = Debug::LOG_VERBOSE;
+		}
 
 		Debug::set_enabled(true);
 		Debug::set_loglevel($xdebug);
@@ -644,9 +662,9 @@ class Feeds extends Handler_Protected {
 		$sth->execute([$feed_id, $_SESSION['uid']]);
 
 		if (!$sth->fetch()) {
-		    print "Access denied.";
-		    return;
-        }
+			print "Access denied.";
+			return;
+		}
 		?>
 		<!DOCTYPE html>
 		<html>
@@ -731,7 +749,10 @@ class Feeds extends Handler_Protected {
 
 	}
 
-	static function _catchup($feed, $cat_view, $owner_uid = false, $mode = 'all', $search = false) {
+	/**
+	 * @param array<int, string> $search
+	 */
+	static function _catchup(string $feed_id_or_tag_name, bool $cat_view, ?int $owner_uid = null, string $mode = 'all', ?array $search = null): void {
 
 		if (!$owner_uid) $owner_uid = $_SESSION['uid'];
 
@@ -785,14 +806,16 @@ class Feeds extends Handler_Protected {
 				$date_qpart = "true";
 		}
 
-		if (is_numeric($feed)) {
+		if (is_numeric($feed_id_or_tag_name)) {
+			$feed_id = (int) $feed_id_or_tag_name;
+
 			if ($cat_view) {
 
-				if ($feed >= 0) {
+				if ($feed_id >= 0) {
 
-					if ($feed > 0) {
-						$children = self::_get_child_cats($feed, $owner_uid);
-						array_push($children, $feed);
+					if ($feed_id > 0) {
+						$children = self::_get_child_cats($feed_id, $owner_uid);
+						array_push($children, $feed_id);
 						$children = array_map("intval", $children);
 
 						$children = join(",", $children);
@@ -810,7 +833,7 @@ class Feeds extends Handler_Protected {
 										(SELECT id FROM ttrss_feeds WHERE $cat_qpart) AND $date_qpart AND $search_qpart) as tmp)");
 					$sth->execute([$owner_uid]);
 
-				} else if ($feed == -2) {
+				} else if ($feed_id == -2) {
 
 					$sth = $pdo->prepare("UPDATE ttrss_user_entries
 						SET unread = false,last_read = NOW() WHERE (SELECT COUNT(*)
@@ -819,18 +842,18 @@ class Feeds extends Handler_Protected {
 					$sth->execute([$owner_uid]);
 				}
 
-			} else if ($feed > 0) {
+			} else if ($feed_id > 0) {
 
 				$sth = $pdo->prepare("UPDATE ttrss_user_entries
 					SET unread = false, last_read = NOW() WHERE ref_id IN
 						(SELECT id FROM
 							(SELECT DISTINCT id FROM ttrss_entries, ttrss_user_entries WHERE ref_id = id
 								AND owner_uid = ? AND unread = true AND feed_id = ? AND $date_qpart AND $search_qpart) as tmp)");
-				$sth->execute([$owner_uid, $feed]);
+				$sth->execute([$owner_uid, $feed_id]);
 
-			} else if ($feed < 0 && $feed > LABEL_BASE_INDEX) { // special, like starred
+			} else if ($feed_id < 0 && $feed_id > LABEL_BASE_INDEX) { // special, like starred
 
-				if ($feed == -1) {
+				if ($feed_id == -1) {
 					$sth = $pdo->prepare("UPDATE ttrss_user_entries
 						SET unread = false, last_read = NOW() WHERE ref_id IN
 							(SELECT id FROM
@@ -839,7 +862,7 @@ class Feeds extends Handler_Protected {
 					$sth->execute([$owner_uid]);
 				}
 
-				if ($feed == -2) {
+				if ($feed_id == -2) {
 					$sth = $pdo->prepare("UPDATE ttrss_user_entries
 						SET unread = false, last_read = NOW() WHERE ref_id IN
 							(SELECT id FROM
@@ -848,7 +871,7 @@ class Feeds extends Handler_Protected {
 					$sth->execute([$owner_uid]);
 				}
 
-				if ($feed == -3) {
+				if ($feed_id == -3) {
 
 					$intl = (int) get_pref(Prefs::FRESH_ARTICLE_MAX_AGE);
 
@@ -867,7 +890,7 @@ class Feeds extends Handler_Protected {
 					$sth->execute([$owner_uid]);
 				}
 
-				if ($feed == -4) {
+				if ($feed_id == -4) {
 					$sth = $pdo->prepare("UPDATE ttrss_user_entries
 						SET unread = false, last_read = NOW() WHERE ref_id IN
 							(SELECT id FROM
@@ -875,10 +898,9 @@ class Feeds extends Handler_Protected {
 									AND owner_uid = ? AND unread = true AND $date_qpart AND $search_qpart) as tmp)");
 					$sth->execute([$owner_uid]);
 				}
+			} else if ($feed_id < LABEL_BASE_INDEX) { // label
 
-			} else if ($feed < LABEL_BASE_INDEX) { // label
-
-				$label_id = Labels::feed_to_label_id($feed);
+				$label_id = Labels::feed_to_label_id($feed_id);
 
 				$sth = $pdo->prepare("UPDATE ttrss_user_entries
 					SET unread = false, last_read = NOW() WHERE ref_id IN
@@ -887,23 +909,21 @@ class Feeds extends Handler_Protected {
 								AND label_id = ? AND ref_id = article_id
 								AND owner_uid = ? AND unread = true AND $date_qpart AND $search_qpart) as tmp)");
 				$sth->execute([$label_id, $owner_uid]);
-
 			}
-
 		} else { // tag
+			$tag_name = $feed_id_or_tag_name;
+
 			$sth = $pdo->prepare("UPDATE ttrss_user_entries
 				SET unread = false, last_read = NOW() WHERE ref_id IN
 					(SELECT id FROM
 						(SELECT DISTINCT ttrss_entries.id FROM ttrss_entries, ttrss_user_entries, ttrss_tags WHERE ref_id = ttrss_entries.id
 							AND post_int_id = int_id AND tag_name = ?
 							AND ttrss_user_entries.owner_uid = ? AND unread = true AND $date_qpart AND $search_qpart) as tmp)");
-			$sth->execute([$feed, $owner_uid]);
-
+			$sth->execute([$tag_name, $owner_uid]);
 		}
 	}
 
-	static function _get_counters($feed, $is_cat = false, $unread_only = false,
-							 $owner_uid = false) {
+	static function _get_counters(int $feed, bool $is_cat = false, bool $unread_only = false, ?int $owner_uid = null): int {
 
 		$n_feed = (int) $feed;
 		$need_entries = false;
@@ -1002,7 +1022,7 @@ class Feeds extends Handler_Protected {
 		}
 	}
 
-	function add() {
+	function add(): void {
 		$feed = clean($_REQUEST['feed']);
 		$cat = clean($_REQUEST['cat'] ?? '');
 		$need_auth = isset($_REQUEST['need_auth']);
@@ -1015,7 +1035,7 @@ class Feeds extends Handler_Protected {
 	}
 
 	/**
-	 * @return array (code => Status code, message => error message if available)
+	 * @return array<string, mixed> (code => Status code, message => error message if available)
 	 *
 	 *                 0 - OK, Feed already exists
 	 *                 1 - OK, Feed added
@@ -1029,8 +1049,7 @@ class Feeds extends Handler_Protected {
 	 *                 7 - Error while creating feed database entry.
 	 *                 8 - Permission denied (ACCESS_LEVEL_READONLY).
 	 */
-	static function _subscribe($url, $cat_id = 0,
-							   $auth_login = '', $auth_pass = '') : array {
+	static function _subscribe(string $url, int $cat_id = 0, string $auth_login = '', string $auth_pass = ''): array {
 
 		$user = ORM::for_table("ttrss_users")->find_one($_SESSION['uid']);
 
@@ -1109,15 +1128,18 @@ class Feeds extends Handler_Protected {
 		}
 	}
 
-	static function _get_icon_file($feed_id) {
+	static function _get_icon_file(int $feed_id): string {
 		return Config::get(Config::ICONS_DIR) . "/$feed_id.ico";
 	}
 
-	static function _has_icon($id) {
+	static function _has_icon(int $id): bool {
 		return is_file(Config::get(Config::ICONS_DIR) . "/$id.ico") && filesize(Config::get(Config::ICONS_DIR) . "/$id.ico") > 0;
 	}
 
-	static function _get_icon($id) {
+	/**
+	 * @return false|string false if the icon ID was unrecognized, otherwise, the icon identifier string
+	 */
+	static function _get_icon(int $id) {
 		switch ($id) {
 			case 0:
 				return "archive";
@@ -1137,7 +1159,7 @@ class Feeds extends Handler_Protected {
 				} else {
 					$icon = self::_get_icon_file($id);
 
-                    if ($icon && file_exists($icon)) {
+					if ($icon && file_exists($icon)) {
 						return Config::get(Config::ICONS_URL) . "/" . basename($icon) . "?" . filemtime($icon);
 					}
 				}
@@ -1147,6 +1169,9 @@ class Feeds extends Handler_Protected {
 		return false;
 	}
 
+	/**
+	 * @return false|int false if the feed couldn't be found by URL+owner, otherwise the feed ID
+	 */
 	static function _find_by_url(string $feed_url, int $owner_uid) {
 		$feed = ORM::for_table('ttrss_feeds')
 			->where('owner_uid', $owner_uid)
@@ -1160,7 +1185,11 @@ class Feeds extends Handler_Protected {
 		}
 	}
 
-	/** $owner_uid defaults to $_SESSION['uid] */
+	/**
+	 * $owner_uid defaults to $_SESSION['uid']
+	 *
+	 * @return false|int false if the category/feed couldn't be found by title, otherwise its ID
+	 */
 	static function _find_by_title(string $title, bool $cat = false, int $owner_uid = 0) {
 
 		$res = false;
@@ -1184,8 +1213,8 @@ class Feeds extends Handler_Protected {
 		}
 	}
 
-	static function _get_title($id, bool $cat = false) {
-	    $pdo = Db::pdo();
+	static function _get_title(int $id, bool $cat = false): string {
+		$pdo = Db::pdo();
 
 		if ($cat) {
 			return self::_get_cat_title($id);
@@ -1197,7 +1226,7 @@ class Feeds extends Handler_Protected {
 			return __("Fresh articles");
 		} else if ($id == -4) {
 			return __("All articles");
-		} else if ($id === 0 || $id === "0") {
+		} else if ($id === 0) {
 			return __("Archived articles");
 		} else if ($id == -6) {
 			return __("Recently read");
@@ -1226,12 +1255,12 @@ class Feeds extends Handler_Protected {
 			}
 
 		} else {
-			return $id;
+			return "$id";
 		}
 	}
 
 	// only real cats
-	static function _get_cat_marked(int $cat, int $owner_uid = 0) {
+	static function _get_cat_marked(int $cat, int $owner_uid = 0): int {
 
 		if (!$owner_uid) $owner_uid = $_SESSION["uid"];
 
@@ -1245,16 +1274,17 @@ class Feeds extends Handler_Protected {
                     WHERE (cat_id = :cat OR (:cat IS NULL AND cat_id IS NULL))
 					AND owner_uid = :uid)
 				AND owner_uid = :uid");
+
 			$sth->execute(["cat" => $cat ? $cat : null, "uid" => $owner_uid]);
-			$row = $sth->fetch();
 
-			return $row["marked"];
-		} else {
-			return 0;
+			if ($row = $sth->fetch()) {
+				return (int) $row["marked"];
+			}
 		}
+		return 0;
 	}
 
-	static function _get_cat_unread(int $cat, int $owner_uid = 0) {
+	static function _get_cat_unread(int $cat, int $owner_uid = 0): int {
 
 		if (!$owner_uid) $owner_uid = $_SESSION["uid"];
 
@@ -1265,14 +1295,15 @@ class Feeds extends Handler_Protected {
 			$sth = $pdo->prepare("SELECT SUM(CASE WHEN unread THEN 1 ELSE 0 END) AS unread
 				FROM ttrss_user_entries
 				WHERE feed_id IN (SELECT id FROM ttrss_feeds
-                    WHERE (cat_id = :cat OR (:cat IS NULL AND cat_id IS NULL))
+					WHERE (cat_id = :cat OR (:cat IS NULL AND cat_id IS NULL))
 					AND owner_uid = :uid)
 				AND owner_uid = :uid");
-			$sth->execute(["cat" => $cat ? $cat : null, "uid" => $owner_uid]);
-			$row = $sth->fetch();
 
-			return $row["unread"];
+			$sth->execute(["cat" => $cat ? $cat : null, "uid" => $owner_uid]);
 
+			if ($row = $sth->fetch()) {
+				return (int) $row["unread"];
+			}
 		} else if ($cat == -1) {
 			return 0;
 		} else if ($cat == -2) {
@@ -1280,15 +1311,19 @@ class Feeds extends Handler_Protected {
 			$sth = $pdo->prepare("SELECT COUNT(DISTINCT article_id) AS unread
 				FROM ttrss_user_entries ue, ttrss_user_labels2 l
 				WHERE article_id = ref_id AND unread IS true AND ue.owner_uid = :uid");
+
 			$sth->execute(["uid" => $owner_uid]);
-            $row = $sth->fetch();
 
-			return $row["unread"];
+			if ($row = $sth->fetch()) {
+				return (int) $row["unread"];
+			}
 		}
+
+		return 0;
 	}
 
 	// only accepts real cats (>= 0)
-	static function _get_cat_children_unread(int $cat, int $owner_uid = 0) {
+	static function _get_cat_children_unread(int $cat, int $owner_uid = 0): int {
 		if (!$owner_uid) $owner_uid = $_SESSION["uid"];
 
 		$pdo = Db::pdo();
@@ -1307,7 +1342,7 @@ class Feeds extends Handler_Protected {
 		return $unread;
 	}
 
-	static function _get_global_unread(int $user_id = 0) {
+	static function _get_global_unread(int $user_id = 0): int {
 
 		if (!$user_id) $user_id = $_SESSION["uid"];
 
@@ -1323,7 +1358,7 @@ class Feeds extends Handler_Protected {
 		return $row["count"];
 	}
 
-	static function _get_cat_title(int $cat_id) {
+	static function _get_cat_title(int $cat_id): string {
 		switch ($cat_id) {
 			case 0:
 				return __("Uncategorized");
@@ -1343,7 +1378,7 @@ class Feeds extends Handler_Protected {
 		}
 	}
 
-	private static function _get_label_unread($label_id, int $owner_uid = 0) {
+	private static function _get_label_unread(int $label_id, ?int $owner_uid = null): int {
 		if (!$owner_uid) $owner_uid = $_SESSION["uid"];
 
 		$pdo = Db::pdo();
@@ -1360,7 +1395,11 @@ class Feeds extends Handler_Protected {
 		}
 	}
 
-	static function _get_headlines($params) {
+	/**
+	 * @param array<string, mixed> $params
+	 * @return array<int, mixed> $result, $feed_title, $feed_site_url, $last_error, $last_updated, $highlight_words, $first_id, $is_vfeed, $query_error_override
+	 */
+	static function _get_headlines($params): array {
 
 		$pdo = Db::pdo();
 
@@ -1577,7 +1616,7 @@ class Feeds extends Handler_Protected {
 		} else if ($feed <= LABEL_BASE_INDEX) { // labels
 			$label_id = Labels::feed_to_label_id($feed);
 
-			$query_strategy_part = "label_id = ".$pdo->quote($label_id)." AND
+			$query_strategy_part = "label_id = $label_id AND
 					ttrss_labels2.id = ttrss_user_labels2.label_id AND
 					ttrss_user_labels2.article_id = ref_id";
 
@@ -1857,7 +1896,10 @@ class Feeds extends Handler_Protected {
 
 	}
 
-	static function _get_parent_cats(int $cat, int $owner_uid) {
+	/**
+	 * @return array<int, int>
+	 */
+	static function _get_parent_cats(int $cat, int $owner_uid): array {
 		$rv = array();
 
 		$pdo = Db::pdo();
@@ -1874,7 +1916,10 @@ class Feeds extends Handler_Protected {
 		return $rv;
 	}
 
-	static function _get_child_cats(int $cat, int $owner_uid) {
+	/**
+	 * @return array<int, int>
+	 */
+	static function _get_child_cats(int $cat, int $owner_uid): array {
 		$rv = array();
 
 		$pdo = Db::pdo();
@@ -1891,7 +1936,11 @@ class Feeds extends Handler_Protected {
 		return $rv;
 	}
 
-	static function _cats_of(array $feeds, int $owner_uid, bool $with_parents = false) {
+	/**
+	 * @param array<int, int> $feeds
+	 * @return array<int, int>
+	 */
+	static function _cats_of(array $feeds, int $owner_uid, bool $with_parents = false): array {
 		if (count($feeds) == 0)
 			return [];
 
@@ -1930,24 +1979,27 @@ class Feeds extends Handler_Protected {
 		}
 	}
 
-    private function _color_of($name) {
-        $colormap = [ "#1cd7d7","#d91111","#1212d7","#8e16e5","#7b7b7b",
-            "#39f110","#0bbea6","#ec0e0e","#1534f2","#b9e416",
-            "#479af2","#f36b14","#10c7e9","#1e8fe7","#e22727" ];
+	private function _color_of(string $name): string {
+		$colormap = [ "#1cd7d7","#d91111","#1212d7","#8e16e5","#7b7b7b",
+			"#39f110","#0bbea6","#ec0e0e","#1534f2","#b9e416",
+			"#479af2","#f36b14","#10c7e9","#1e8fe7","#e22727" ];
 
-        $sum = 0;
+		$sum = 0;
 
-        for ($i = 0; $i < strlen($name); $i++) {
-            $sum += ord($name[$i]);
-        }
+		for ($i = 0; $i < strlen($name); $i++) {
+			$sum += ord($name[$i]);
+		}
 
-        $sum %= count($colormap);
+		$sum %= count($colormap);
 
-        return $colormap[$sum];
+		return $colormap[$sum];
 	}
 
-	private static function _get_feeds_from_html($url, $content) {
-		$url     = UrlHelper::validate($url);
+	/**
+	 * @return array<string, string> array of feed URL -> feed title
+	 */
+	private static function _get_feeds_from_html(string $url, string $content): array {
+		$url = UrlHelper::validate($url);
 		$baseUrl = substr($url, 0, strrpos($url, '/') + 1);
 
 		$feedUrls = [];
@@ -1964,9 +2016,7 @@ class Feeds extends Handler_Protected {
 					if ($title == '') {
 						$title = $entry->getAttribute('type');
 					}
-					$feedUrl = rewrite_relative_url(
-						$baseUrl, $entry->getAttribute('href')
-					);
+					$feedUrl = UrlHelper::rewrite_relative($baseUrl, $entry->getAttribute('href'));
 					$feedUrls[$feedUrl] = $title;
 				}
 			}
@@ -1974,11 +2024,11 @@ class Feeds extends Handler_Protected {
 		return $feedUrls;
 	}
 
-	static function _is_html($content) {
+	static function _is_html(string $content): bool {
 		return preg_match("/<html|DOCTYPE html/i", substr($content, 0, 8192)) !== 0;
 	}
 
-	static function _remove_cat(int $id, int $owner_uid) {
+	static function _remove_cat(int $id, int $owner_uid): void {
 		$cat = ORM::for_table('ttrss_feed_categories')
 			->where('owner_uid', $owner_uid)
 			->find_one($id);
@@ -1987,7 +2037,7 @@ class Feeds extends Handler_Protected {
 			$cat->delete();
 	}
 
-	static function _add_cat(string $title, int $owner_uid, int $parent_cat = null, int $order_id = 0) {
+	static function _add_cat(string $title, int $owner_uid, int $parent_cat = null, int $order_id = 0): bool {
 
 		$cat = ORM::for_table('ttrss_feed_categories')
 			->where('owner_uid', $owner_uid)
@@ -2011,13 +2061,18 @@ class Feeds extends Handler_Protected {
 		return false;
 	}
 
-	static function _clear_access_keys(int $owner_uid) {
+	static function _clear_access_keys(int $owner_uid): void {
 		$key = ORM::for_table('ttrss_access_keys')
 			->where('owner_uid', $owner_uid)
 			->delete_many();
 	}
 
-	static function _update_access_key(string $feed_id, bool $is_cat, int $owner_uid) {
+	/**
+	 * @param string $feed_id may be a feed ID or tag
+	 *
+	 * @see Handler_Public#generate_syndicated_feed()
+	 */
+	static function _update_access_key(string $feed_id, bool $is_cat, int $owner_uid): ?string {
 		$key = ORM::for_table('ttrss_access_keys')
 			->where('owner_uid', $owner_uid)
 			->where('feed_id', $feed_id)
@@ -2027,7 +2082,12 @@ class Feeds extends Handler_Protected {
 		return self::_get_access_key($feed_id, $is_cat, $owner_uid);
 	}
 
-	static function _get_access_key(string $feed_id, bool $is_cat, int $owner_uid) {
+	/**
+	 * @param string $feed_id may be a feed ID or tag
+	 *
+	 * @see Handler_Public#generate_syndicated_feed()
+	 */
+	static function _get_access_key(string $feed_id, bool $is_cat, int $owner_uid): ?string {
 		$key = ORM::for_table('ttrss_access_keys')
 			->where('owner_uid', $owner_uid)
 			->where('feed_id', $feed_id)
@@ -2036,21 +2096,23 @@ class Feeds extends Handler_Protected {
 
 		if ($key) {
 			return $key->access_key;
-		} else {
-			$key = ORM::for_table('ttrss_access_keys')->create();
+		}
 
-			$key->owner_uid = $owner_uid;
-			$key->feed_id = $feed_id;
-			$key->is_cat = $is_cat;
-			$key->access_key = uniqid_short();
+		$key = ORM::for_table('ttrss_access_keys')->create();
 
-			if ($key->save()) {
-				return $key->access_key;
-			}
+		$key->owner_uid = $owner_uid;
+		$key->feed_id = $feed_id;
+		$key->is_cat = $is_cat;
+		$key->access_key = uniqid_short();
+
+		if ($key->save()) {
+			return $key->access_key;
 		}
+
+		return null;
 	}
 
-	static function _purge(int $feed_id, int $purge_interval) {
+	static function _purge(int $feed_id, int $purge_interval): ?int {
 
 		if (!$purge_interval) $purge_interval = self::_get_purge_interval($feed_id);
 
@@ -2079,7 +2141,7 @@ class Feeds extends Handler_Protected {
 
 			if ($purge_interval <= 0) {
 				Debug::log("purge_feed: purging disabled for this feed, nothing to do.", Debug::$LOG_VERBOSE);
-				return;
+				return null;
 			}
 
 			if (!$purge_unread)
@@ -2120,7 +2182,7 @@ class Feeds extends Handler_Protected {
 		return $rows_deleted;
 	}
 
-	private static function _get_purge_interval(int $feed_id) {
+	private static function _get_purge_interval(int $feed_id): int {
 		$feed = ORM::for_table('ttrss_feeds')->find_one($feed_id);
 
 		if ($feed) {
@@ -2133,7 +2195,10 @@ class Feeds extends Handler_Protected {
 		}
 	}
 
-	private static function _search_to_sql($search, $search_language, $owner_uid) {
+	/**
+	 * @return array{0: string, 1: array<int, string>} [$search_query_part, $search_words]
+	 */
+	private static function _search_to_sql(string $search, string $search_language, int $owner_uid): array {
 		$keywords = str_getcsv(preg_replace('/(-?\w+)\:"(\w+)/', '"${1}:${2}', trim($search)), ' ');
 		$query_keywords = array();
 		$search_words = array();
@@ -2226,7 +2291,7 @@ class Feeds extends Handler_Protected {
 							array_push($query_keywords, "($not
 								(ttrss_entries.id IN (
 									SELECT article_id FROM ttrss_user_labels2 WHERE
-										label_id = ".$pdo->quote($label_id).")))");
+										label_id = $label_id)))");
 						} else {
 							array_push($query_keywords, "(false)");
 						}
@@ -2300,7 +2365,10 @@ class Feeds extends Handler_Protected {
 		return array($search_query_part, $search_words);
 	}
 
-	static function _order_to_override_query($order) {
+	/**
+	 * @return array{0: string, 1: bool}
+	 */
+	static function _order_to_override_query(string $order): array {
 		$query = "";
 		$skip_first_id = false;
 
@@ -2310,7 +2378,9 @@ class Feeds extends Handler_Protected {
 			},
 			$order);
 
-		if ($query)	return [$query, $skip_first_id];
+		if (is_string($query) && $query !== "") {
+			return [$query, $skip_first_id];
+		}
 
 		switch ($order) {
 			case "title":
@@ -2328,7 +2398,7 @@ class Feeds extends Handler_Protected {
 		return [$query, $skip_first_id];
 	}
 
-	private function _mark_timestamp($label) {
+	private function _mark_timestamp(string $label): void {
 
 		if (empty($_REQUEST['timestamps']))
 			return;
diff --git a/classes/handler.php b/classes/handler.php
index 09557c284..3ee42cedb 100644
--- a/classes/handler.php
+++ b/classes/handler.php
@@ -1,22 +1,29 @@
 <?php
 class Handler implements IHandler {
+	// TODO: class properties can be switched to PHP typing if/when the minimum PHP_VERSION is raised to 7.4.0+
+	/** @var PDO */
 	protected $pdo;
+
+	/** @var array<int|string, mixed> */
 	protected $args;
 
-	function __construct($args) {
+	/**
+	 * @param array<int|string, mixed> $args
+	 */
+	function __construct(array $args) {
 		$this->pdo = Db::pdo();
 		$this->args = $args;
 	}
 
-	function csrf_ignore($method) {
+	function csrf_ignore(string $method): bool {
 		return false;
 	}
 
-	function before($method) {
+	function before(string $method): bool {
 		return true;
 	}
 
-	function after() {
+	function after(): bool {
 		return true;
 	}
 
diff --git a/classes/handler/administrative.php b/classes/handler/administrative.php
index f2f5b36ba..533cb3630 100644
--- a/classes/handler/administrative.php
+++ b/classes/handler/administrative.php
@@ -1,6 +1,6 @@
 <?php
 class Handler_Administrative extends Handler_Protected {
-   function before($method) {
+   function before(string $method): bool {
       if (parent::before($method)) {
          if (($_SESSION["access_level"] ?? 0) >= UserHelper::ACCESS_LEVEL_ADMIN) {
             return true;
diff --git a/classes/handler/protected.php b/classes/handler/protected.php
index 8e9e5ca1d..a15fc0956 100644
--- a/classes/handler/protected.php
+++ b/classes/handler/protected.php
@@ -1,7 +1,7 @@
 <?php
 class Handler_Protected extends Handler {
 
-	function before($method) {
+	function before(string $method): bool {
 		return parent::before($method) && !empty($_SESSION['uid']);
 	}
 }
diff --git a/classes/handler/public.php b/classes/handler/public.php
index 14474d0bb..b5282c222 100755
--- a/classes/handler/public.php
+++ b/classes/handler/public.php
@@ -1,10 +1,12 @@
 <?php
 class Handler_Public extends Handler {
 
-	// $feed may be a tag
+	/**
+	 * @param string $feed may be a feed ID or tag
+	 */
 	private function generate_syndicated_feed(int $owner_uid, string $feed, bool $is_cat,
 		int $limit, int $offset, string $search, string $view_mode = "",
-		string $format = 'atom', string $order = "", string $orig_guid = "", string $start_ts = "") {
+		string $format = 'atom', string $order = "", string $orig_guid = "", string $start_ts = ""): void {
 
 		$note_style = 	"background-color : #fff7d5;
 			border-width : 1px; ".
@@ -52,11 +54,13 @@ class Handler_Public extends Handler {
 				PluginHost::feed_to_pfeed_id((int)$feed));
 
 			if ($handler) {
+				// 'get_headlines' is implemented by the plugin.
+				// @phpstan-ignore-next-line
 				$qfh_ret = $handler->get_headlines(PluginHost::feed_to_pfeed_id((int)$feed), $params);
 			} else {
 				user_error("Failed to find handler for plugin feed ID: $feed", E_USER_ERROR);
 
-				return false;
+				return;
 			}
 
 		} else {
@@ -89,11 +93,13 @@ class Handler_Public extends Handler {
 				$line["content_preview"] = Sanitizer::sanitize(truncate_string(strip_tags($line["content"]), 100, '...'));
 				$line["tags"] = Article::_get_tags($line["id"], $owner_uid);
 
+				$max_excerpt_length = 250;
+
 				PluginHost::getInstance()->chain_hooks_callback(PluginHost::HOOK_QUERY_HEADLINES,
 					function ($result) use (&$line) {
 						$line = $result;
 					},
-					$line);
+					$line, $max_excerpt_length);
 
 				PluginHost::getInstance()->chain_hooks_callback(PluginHost::HOOK_ARTICLE_EXPORT_FEED,
 					function ($result) use (&$line) {
@@ -109,7 +115,7 @@ class Handler_Public extends Handler {
 				$tpl->setVariable('ARTICLE_EXCERPT', $line["content_preview"], true);
 
 				$content = Sanitizer::sanitize($line["content"], false, $owner_uid,
-					$feed_site_url, false, $line["id"]);
+					$feed_site_url, null, $line["id"]);
 
 				$content = DiskCache::rewrite_urls($content);
 
@@ -207,7 +213,7 @@ class Handler_Public extends Handler {
 				$article['link']	= $line['link'];
 				$article['title'] = $line['title'];
 				$article['excerpt'] = $line["content_preview"];
-				$article['content'] = Sanitizer::sanitize($line["content"], false, $owner_uid, $feed_site_url, false, $line["id"]);
+				$article['content'] = Sanitizer::sanitize($line["content"], false, $owner_uid, $feed_site_url, null, $line["id"]);
 				$article['updated'] = date('c', strtotime($line["updated"]));
 
 				if (!empty($line['note'])) $article['note'] = $line['note'];
@@ -247,7 +253,7 @@ class Handler_Public extends Handler {
 		}
 	}
 
-	function getUnread() {
+	function getUnread(): void {
 		$login = clean($_REQUEST["login"]);
 		$fresh = clean($_REQUEST["fresh"]) == "1";
 
@@ -265,7 +271,7 @@ class Handler_Public extends Handler {
 		}
 	}
 
-	function getProfiles() {
+	function getProfiles(): void {
 		$login = clean($_REQUEST["login"]);
 		$rv = [];
 
@@ -288,7 +294,7 @@ class Handler_Public extends Handler {
 		print json_encode($rv);
 	}
 
-	function logout() {
+	function logout(): void {
 		if (validate_csrf($_POST["csrf_token"])) {
 			UserHelper::logout();
 			header("Location: index.php");
@@ -298,7 +304,7 @@ class Handler_Public extends Handler {
 		}
 	}
 
-	function rss() {
+	function rss(): void {
 		$feed = clean($_REQUEST["id"]);
 		$key = clean($_REQUEST["key"]);
 		$is_cat = clean($_REQUEST["is_cat"] ?? false);
@@ -333,21 +339,21 @@ class Handler_Public extends Handler {
 		header('HTTP/1.1 403 Forbidden');
 	}
 
-	function updateTask() {
+	function updateTask(): void {
 		PluginHost::getInstance()->run_hooks(PluginHost::HOOK_UPDATE_TASK);
 	}
 
-	function housekeepingTask() {
+	function housekeepingTask(): void {
 		PluginHost::getInstance()->run_hooks(PluginHost::HOOK_HOUSE_KEEPING);
 	}
 
-	function globalUpdateFeeds() {
+	function globalUpdateFeeds(): void {
 		RPC::updaterandomfeed_real();
 
 		PluginHost::getInstance()->run_hooks(PluginHost::HOOK_UPDATE_TASK);
 	}
 
-	function login() {
+	function login(): void {
 		if (!Config::get(Config::SINGLE_USER_MODE)) {
 
 			$login = clean($_POST["login"]);
@@ -403,12 +409,12 @@ class Handler_Public extends Handler {
 		}
 	}
 
-	function index() {
+	function index(): void {
 		header("Content-Type: text/plain");
 		print Errors::to_json(Errors::E_UNKNOWN_METHOD);
 	}
 
-	function forgotpass() {
+	function forgotpass(): void {
 		startup_gettext();
 		session_start();
 
@@ -587,7 +593,7 @@ class Handler_Public extends Handler {
 		print "</html>";
 	}
 
-	function dbupdate() {
+	function dbupdate(): void {
 		startup_gettext();
 
 		if (!Config::get(Config::SINGLE_USER_MODE) && ($_SESSION["access_level"] ?? 0) < 10) {
@@ -730,7 +736,7 @@ class Handler_Public extends Handler {
 		<?php
 	}
 
-	function cached() {
+	function cached(): void {
 		list ($cache_dir, $filename) = explode("/", $_GET["file"], 2);
 
 		// we do not allow files with extensions at the moment
@@ -746,7 +752,7 @@ class Handler_Public extends Handler {
 		}
 	}
 
-	private function _make_article_tag_uri($id, $timestamp) {
+	private function _make_article_tag_uri(int $id, string $timestamp): string {
 
 		$timestamp = date("Y-m-d", strtotime($timestamp));
 
@@ -756,7 +762,7 @@ class Handler_Public extends Handler {
 	// this should be used very carefully because this endpoint is exposed to unauthenticated users
 	// plugin data is not loaded because there's no user context and owner_uid/session may or may not be available
 	// in general, don't do anything user-related in here and do not modify $_SESSION
-	public function pluginhandler() {
+	public function pluginhandler(): void {
 		$host = new PluginHost();
 
 		$plugin_name = basename(clean($_REQUEST["plugin"]));
@@ -788,7 +794,7 @@ class Handler_Public extends Handler {
 		}
 	}
 
-	static function _render_login_form(string $return_to = "") {
+	static function _render_login_form(string $return_to = ""): void {
 		header('Cache-Control: public');
 
 		if ($return_to)
diff --git a/classes/iauthmodule.php b/classes/iauthmodule.php
index e714cc6ca..dbf8c5587 100644
--- a/classes/iauthmodule.php
+++ b/classes/iauthmodule.php
@@ -1,5 +1,18 @@
 <?php
 interface IAuthModule {
-	function authenticate($login, $password); // + optional third parameter: $service
-	function hook_auth_user(...$args); // compatibility wrapper due to how hooks work
+	/**
+	 * @param string $login
+	 * @param string $password
+	 * @param string $service
+	 * @return int|false user_id
+	 */
+	function authenticate($login, $password, $service = '');
+
+	/** this is a pluginhost compatibility wrapper that invokes $this->authenticate(...$args) (Auth_Base)
+ 	 * @param string $login
+	 * @param string $password
+	 * @param string $service
+	 * @return int|false user_id
+	 */
+	function hook_auth_user($login, $password, $service = '');
 }
diff --git a/classes/ihandler.php b/classes/ihandler.php
index 01c9e3109..215143370 100644
--- a/classes/ihandler.php
+++ b/classes/ihandler.php
@@ -1,6 +1,6 @@
 <?php
 interface IHandler {
-	function csrf_ignore($method);
-	function before($method);
-	function after();
+	function csrf_ignore(string $method): bool;
+	function before(string $method): bool;
+	function after(): bool;
 }
diff --git a/classes/labels.php b/classes/labels.php
index 570f24f4f..026e6621f 100644
--- a/classes/labels.php
+++ b/classes/labels.php
@@ -1,15 +1,15 @@
 <?php
 class Labels
 {
-	static function label_to_feed_id($label) {
+	static function label_to_feed_id(int $label): int {
 		return LABEL_BASE_INDEX - 1 - abs($label);
 	}
 
-	static function feed_to_label_id($feed) {
+	static function feed_to_label_id(int $feed): int {
 		return LABEL_BASE_INDEX - 1 + abs($feed);
 	}
 
-	static function find_id($label, $owner_uid) {
+	static function find_id(string $label, int $owner_uid): int {
 		$pdo = Db::pdo();
 
 		$sth = $pdo->prepare("SELECT id FROM ttrss_labels2 WHERE LOWER(caption) = LOWER(?)
@@ -23,7 +23,7 @@ class Labels
 		}
 	}
 
-	static function find_caption($label, $owner_uid) {
+	static function find_caption(int $label, int $owner_uid): string {
 		$pdo = Db::pdo();
 
 		$sth = $pdo->prepare("SELECT caption FROM ttrss_labels2 WHERE id = ?
@@ -37,18 +37,24 @@ class Labels
 		}
 	}
 
-	static function get_as_hash($owner_uid) {
+	/**
+	 * @return array<int, array<string, string>>
+	 */
+	static function get_as_hash(int $owner_uid): array {
 		$rv = [];
 		$labels = Labels::get_all($owner_uid);
 
 		foreach ($labels as $i => $label) {
-			$rv[$label["id"]] = $labels[$i];
+			$rv[(int)$label["id"]] = $labels[$i];
 		}
 
 		return $rv;
 	}
 
-	static function get_all($owner_uid)	{
+	/**
+	 * @return array<int, array<string, string>> An array of label detail arrays
+	 */
+	static function get_all(int $owner_uid)	{
 		$rv = array();
 
 		$pdo = Db::pdo();
@@ -64,7 +70,13 @@ class Labels
 		return $rv;
 	}
 
-	static function update_cache($owner_uid, $id, $labels = false, $force = false) {
+	/**
+	 * @param array{'no-labels': 1}|array<int, array<int, array{0: int, 1: string, 2: string, 3: string}>> $labels
+	 * [label_id, caption, fg_color, bg_color]
+	 *
+	 * @see Article::_get_labels()
+	 */
+	static function update_cache(int $owner_uid, int $id, array $labels, bool $force = false): void {
 		$pdo = Db::pdo();
 
 		if ($force)
@@ -81,7 +93,7 @@ class Labels
 
 	}
 
-	static function clear_cache($id)	{
+	static function clear_cache(int $id): void	{
 
 		$pdo = Db::pdo();
 
@@ -91,7 +103,7 @@ class Labels
 
 	}
 
-	static function remove_article($id, $label, $owner_uid) {
+	static function remove_article(int $id, string $label, int $owner_uid): void {
 
 		$label_id = self::find_id($label, $owner_uid);
 
@@ -109,7 +121,7 @@ class Labels
 		self::clear_cache($id);
 	}
 
-	static function add_article($id, $label, $owner_uid)	{
+	static function add_article(int $id, string $label, int $owner_uid): void {
 
 		$label_id = self::find_id($label, $owner_uid);
 
@@ -138,7 +150,7 @@ class Labels
 
 	}
 
-	static function remove($id, $owner_uid) {
+	static function remove(int $id, int $owner_uid): void {
 		if (!$owner_uid) $owner_uid = $_SESSION["uid"];
 
 		$pdo = Db::pdo();
@@ -182,7 +194,10 @@ class Labels
 		if (!$tr_in_progress) $pdo->commit();
 	}
 
-	static function create($caption, $fg_color = '', $bg_color = '', $owner_uid = false)	{
+	/**
+	 * @return false|int false if the check for an existing label failed, otherwise the number of rows inserted (1 on success)
+	 */
+	static function create(string $caption, ?string $fg_color = '', ?string $bg_color = '', ?int $owner_uid = null)	{
 
 		if (!$owner_uid) $owner_uid = $_SESSION['uid'];
 
diff --git a/classes/logger.php b/classes/logger.php
index 42ab4452c..ef6173a42 100755
--- a/classes/logger.php
+++ b/classes/logger.php
@@ -1,6 +1,9 @@
 <?php
 class Logger {
+	/** @var Logger|null */
 	private static $instance;
+
+	/** @var Logger_Adapter|null */
 	private $adapter;
 
 	const LOG_DEST_SQL = "sql";
@@ -25,11 +28,11 @@ class Logger {
 		16384		=> 'E_USER_DEPRECATED',
 		32767		=> 'E_ALL'];
 
-	static function log_error(int $errno, string $errstr, string $file, int $line, $context) {
+	static function log_error(int $errno, string $errstr, string $file, int $line, string $context): bool {
 		return self::get_instance()->_log_error($errno, $errstr, $file, $line, $context);
 	}
 
-	private function _log_error($errno, $errstr, $file, $line, $context) {
+	private function _log_error(int $errno, string $errstr, string $file, int $line, string $context): bool {
 		//if ($errno == E_NOTICE) return false;
 
 		if ($this->adapter)
@@ -38,11 +41,11 @@ class Logger {
 			return false;
 	}
 
-	static function log(int $errno, string $errstr, $context = "") {
+	static function log(int $errno, string $errstr, string $context = ""): bool {
 		return self::get_instance()->_log($errno, $errstr, $context);
 	}
 
-	private function _log(int $errno, string $errstr, $context = "") {
+	private function _log(int $errno, string $errstr, string $context = ""): bool {
 		if ($this->adapter)
 			return $this->adapter->log_error($errno, $errstr, '', 0, $context);
 		else
@@ -65,7 +68,7 @@ class Logger {
 			$this->adapter = new Logger_Stdout();
 			break;
 		default:
-			$this->adapter = false;
+			$this->adapter = null;
 		}
 
 		if ($this->adapter && !implements_interface($this->adapter, "Logger_Adapter"))
diff --git a/classes/logger/adapter.php b/classes/logger/adapter.php
index 79f641441..b0287b5fa 100644
--- a/classes/logger/adapter.php
+++ b/classes/logger/adapter.php
@@ -1,4 +1,4 @@
 <?php
 interface Logger_Adapter {
-   function log_error(int $errno, string $errstr, string $file, int $line, $context);
-}
\ No newline at end of file
+   function log_error(int $errno, string $errstr, string $file, int $line, string $context): bool;
+}
diff --git a/classes/logger/sql.php b/classes/logger/sql.php
index 0f6177a5b..5f3c67852 100755
--- a/classes/logger/sql.php
+++ b/classes/logger/sql.php
@@ -10,7 +10,7 @@ class Logger_SQL implements Logger_Adapter {
 		ORM::configure('return_result_sets', true, $conn);
 	}
 
-	function log_error(int $errno, string $errstr, string $file, int $line, $context) {
+	function log_error(int $errno, string $errstr, string $file, int $line, string $context): bool {
 
 		if (Config::get_schema_version() > 117) {
 
diff --git a/classes/logger/stdout.php b/classes/logger/stdout.php
index e906853ce..b15649028 100644
--- a/classes/logger/stdout.php
+++ b/classes/logger/stdout.php
@@ -1,7 +1,7 @@
 <?php
 class Logger_Stdout implements Logger_Adapter {
 
-	function log_error(int $errno, string $errstr, string $file, int $line, $context) {
+	function log_error(int $errno, string $errstr, string $file, int $line, string $context): bool {
 
 		switch ($errno) {
 		case E_ERROR:
@@ -25,6 +25,7 @@ class Logger_Stdout implements Logger_Adapter {
 
 		print "[EEE] $priority $errname ($file:$line) $errstr\n";
 
+		return true;
 	}
 
 }
diff --git a/classes/logger/syslog.php b/classes/logger/syslog.php
index 3ad9858f3..568398ee0 100644
--- a/classes/logger/syslog.php
+++ b/classes/logger/syslog.php
@@ -1,7 +1,7 @@
 <?php
 class Logger_Syslog implements Logger_Adapter {
 
-	function log_error(int $errno, string $errstr, string $file, int $line, $context) {
+	function log_error(int $errno, string $errstr, string $file, int $line, string $context): bool {
 
 		switch ($errno) {
 		case E_ERROR:
@@ -25,6 +25,7 @@ class Logger_Syslog implements Logger_Adapter {
 
 		syslog($priority, "[tt-rss] $errname ($file:$line) $errstr");
 
+		return true;
 	}
 
 }
diff --git a/classes/mailer.php b/classes/mailer.php
index 8238904ee..60b1ce4fd 100644
--- a/classes/mailer.php
+++ b/classes/mailer.php
@@ -1,8 +1,14 @@
 <?php
 class Mailer {
+	// TODO: class properties can be switched to PHP typing if/when the minimum PHP_VERSION is raised to 7.4.0+
+	/** @var string */
 	private $last_error = "";
 
-	function mail($params) {
+	/**
+	 * @param array<string, mixed> $params
+	 * @return bool|int bool if the default mail function handled the request, otherwise an int as described in Mailer#mail()
+	 */
+	function mail(array $params) {
 
 		$to_name = $params["to_name"] ?? "";
 		$to_address = $params["to_address"];
@@ -25,6 +31,8 @@ class Mailer {
 		// 3. any other return value will allow cycling to the next handler and, eventually, to default mail() function
 		// 4. set error message if needed via passed Mailer instance function set_error()
 
+		$hooks_tried = 0;
+
 		foreach (PluginHost::getInstance()->get_hooks(PluginHost::HOOK_SEND_MAIL) as $p) {
 			$rc = $p->hook_send_mail($this, $params);
 
@@ -33,6 +41,8 @@ class Mailer {
 
 			if ($rc == -1)
 				return 0;
+
+			++$hooks_tried;
 		}
 
 		$headers = [ "From: $from_combined", "Content-Type: text/plain; charset=UTF-8" ];
@@ -40,18 +50,18 @@ class Mailer {
 		$rc = mail($to_combined, $subject, $message, implode("\r\n", array_merge($headers, $additional_headers)));
 
 		if (!$rc) {
-			$this->set_error(error_get_last()['message']);
+			$this->set_error(error_get_last()['message'] ?? T_sprintf("Unknown error while sending mail. Hooks tried: %d.", $hooks_tried));
 		}
 
 		return $rc;
 	}
 
-	function set_error($message) {
+	function set_error(string $message): void {
 		$this->last_error = $message;
 		user_error("Error sending mail: $message", E_USER_WARNING);
 	}
 
-	function error() {
+	function error(): string {
 		return $this->last_error;
 	}
 }
diff --git a/classes/opml.php b/classes/opml.php
index e0dbebcda..b9f5f2eab 100644
--- a/classes/opml.php
+++ b/classes/opml.php
@@ -1,12 +1,15 @@
 <?php
 class OPML extends Handler_Protected {
 
-	function csrf_ignore($method) {
+	function csrf_ignore(string $method): bool {
 		$csrf_ignored = array("export", "import");
 
 		return array_search($method, $csrf_ignored) !== false;
 	}
 
+	/**
+	 * @return bool|int|void false if writing the file failed, true if printing succeeded, int if bytes were written to a file, or void if $owner_uid is missing
+	 */
 	function export() {
 		$output_name = sprintf("tt-rss_%s_%s.opml", $_SESSION["name"], date("Y-m-d"));
 		$include_settings = $_REQUEST["include_settings"] == "1";
@@ -17,7 +20,7 @@ class OPML extends Handler_Protected {
 		return $rc;
 	}
 
-	function import() {
+	function import(): void {
 		$owner_uid = $_SESSION["uid"];
 
 		header('Content-Type: text/html; charset=utf-8');
@@ -42,13 +45,11 @@ class OPML extends Handler_Protected {
 			</form>";
 
 		print "</div></body></html>";
-
-
 	}
 
 	// Export
 
-	private function opml_export_category(int $owner_uid, int $cat_id, bool $hide_private_feeds = false, bool $include_settings = true) {
+	private function opml_export_category(int $owner_uid, int $cat_id, bool $hide_private_feeds = false, bool $include_settings = true): string {
 
 		if ($hide_private_feeds)
 			$hide_qpart = "(private IS false AND auth_login = '' AND auth_pass = '')";
@@ -124,6 +125,9 @@ class OPML extends Handler_Protected {
 		return $out;
 	}
 
+	/**
+	 * @return bool|int|void false if writing the file failed, true if printing succeeded, int if bytes were written to a file, or void if $owner_uid is missing
+	 */
 	function opml_export(string $filename, int $owner_uid, bool $hide_private_feeds = false, bool $include_settings = true, bool $file_output = false) {
 		if (!$owner_uid) return;
 
@@ -290,13 +294,14 @@ class OPML extends Handler_Protected {
 
 		if ($file_output)
 			return file_put_contents($filename, $res) > 0;
-		else
-			print $res;
+
+		print $res;
+		return true;
 	}
 
 	// Import
 
-	private function opml_import_feed(DOMNode $node, int $cat_id, int $owner_uid, int $nest) {
+	private function opml_import_feed(DOMNode $node, int $cat_id, int $owner_uid, int $nest): void {
 		$attrs = $node->attributes;
 
 		$feed_title = mb_substr($attrs->getNamedItem('text')->nodeValue, 0, 250);
@@ -341,7 +346,7 @@ class OPML extends Handler_Protected {
 		}
 	}
 
-	private function opml_import_label(DOMNode $node, int $owner_uid, int $nest) {
+	private function opml_import_label(DOMNode $node, int $owner_uid, int $nest): void {
 		$attrs = $node->attributes;
 		$label_name = $attrs->getNamedItem('label-name')->nodeValue;
 
@@ -358,7 +363,7 @@ class OPML extends Handler_Protected {
 		}
 	}
 
-	private function opml_import_preference(DOMNode $node, int $owner_uid, int $nest) {
+	private function opml_import_preference(DOMNode $node, int $owner_uid, int $nest): void {
 		$attrs = $node->attributes;
 		$pref_name = $attrs->getNamedItem('pref-name')->nodeValue;
 
@@ -372,7 +377,7 @@ class OPML extends Handler_Protected {
 		}
 	}
 
-	private function opml_import_filter(DOMNode $node, int $owner_uid, int $nest) {
+	private function opml_import_filter(DOMNode $node, int $owner_uid, int $nest): void {
 		$attrs = $node->attributes;
 
 		$filter_type = $attrs->getNamedItem('filter-type')->nodeValue;
@@ -526,7 +531,7 @@ class OPML extends Handler_Protected {
 		}
 	}
 
-	private function opml_import_category(DOMDocument $doc, ?DOMNode $root_node, int $owner_uid, int $parent_id, int $nest) {
+	private function opml_import_category(DOMDocument $doc, ?DOMNode $root_node, int $owner_uid, int $parent_id, int $nest): void {
 		$default_cat_id = (int) $this->get_feed_category('Imported feeds', $owner_uid, 0);
 
 		if ($root_node) {
@@ -601,6 +606,9 @@ class OPML extends Handler_Protected {
 	}
 
 	/** $filename is optional; assumes HTTP upload with $_FILES otherwise */
+	/**
+	 * @return bool|void false on failure, true if successful, void if $owner_uid is missing
+	 */
 	function opml_import(int $owner_uid, string $filename = "") {
 		if (!$owner_uid) return;
 
@@ -667,7 +675,7 @@ class OPML extends Handler_Protected {
 		return true;
 	}
 
-	private function opml_notice(string $msg, int $prefix_length = 0) {
+	private function opml_notice(string $msg, int $prefix_length = 0): void {
 		if (php_sapi_name() == "cli") {
 			Debug::log(str_repeat("   ", $prefix_length) . $msg);
 		} else {
diff --git a/classes/plugin.php b/classes/plugin.php
index ecafa7888..0a9f837fc 100644
--- a/classes/plugin.php
+++ b/classes/plugin.php
@@ -5,8 +5,14 @@ abstract class Plugin {
 	/** @var PDO $pdo */
 	protected $pdo;
 
-	abstract function init(PluginHost $host);
+	/**
+	 * @param PluginHost $host
+	 *
+	 * @return void
+	 * */
+	abstract function init($host);
 
+	/** @return array<null|float|string|bool> */
 	abstract function about();
 	// return array(1.0, "plugin", "No description", "No author", false);
 
@@ -14,6 +20,7 @@ abstract class Plugin {
 		$this->pdo = Db::pdo();
 	}
 
+	/** @return array<string,bool> */
 	function flags() {
 		/* associative array, possible keys:
 			needs_curl = boolean
@@ -21,36 +28,65 @@ abstract class Plugin {
 		return array();
 	}
 
+	/**
+	 * @param string $method
+	 *
+	 * @return bool */
 	function is_public_method($method) {
 		return false;
 	}
 
+	/**
+	 * @param string $method
+	 *
+	 * @return bool */
 	function csrf_ignore($method) {
 		return false;
 	}
 
+	/** @return string */
 	function get_js() {
 		return "";
 	}
 
+	/** @return string */
+	function get_css() {
+		return "";
+	}
+
+	/** @return string */
 	function get_prefs_js() {
 		return "";
 	}
 
+	/** @return int */
 	function api_version() {
 		return Plugin::API_VERSION_COMPAT;
 	}
 
 	/* gettext-related helpers */
 
+	/**
+	 * @param string $msgid
+	 *
+	 * @return string */
 	function __($msgid) {
+		/** @var Plugin $this -- this is a strictly template-related hack */
 		return _dgettext(PluginHost::object_to_domain($this), $msgid);
 	}
 
+	/**
+	 * @param string $singular
+	 * @param string $plural
+	 * @param int $number
+	 *
+	 * @return string */
 	function _ngettext($singular, $plural, $number) {
+		/** @var Plugin $this -- this is a strictly template-related hack */
 		return _dngettext(PluginHost::object_to_domain($this), $singular, $plural, $number);
 	}
 
+	/** @return string */
 	function T_sprintf() {
 		$args = func_get_args();
 		$msgid = array_shift($args);
@@ -58,4 +94,580 @@ abstract class Plugin {
 		return vsprintf($this->__($msgid), $args);
 	}
 
+	/* plugin hook methods */
+
+	/* GLOBAL hooks are invoked in global context, only available to system plugins (loaded via .env for all users) */
+
+	/** Adds buttons for article (on the right) - e.g. mail, share, add note.
+	 * @param array<string,mixed> $line
+	 * @return string
+	 * @see PluginHost::HOOK_ARTICLE_BUTTON
+	 * @see Plugin::hook_article_left_button()
+	 */
+	function hook_article_button($line) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Allows plugins to alter article data as gathered from feed XML, i.e. embed images, get full text content, etc.
+	 * @param array<string,mixed> $article
+	 * @return array<string,mixed>
+	 * @see PluginHost::HOOK_ARTICLE_FILTER
+	 */
+	function hook_article_filter($article) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return [];
+	}
+
+	/** Allow adding new UI elements (e.g. accordion panes) to (top) tab contents in Preferences
+	 * @param string $tab
+	 * @return void
+	 * @see PluginHost::HOOK_PREFS_TAB
+	 */
+	function hook_prefs_tab($tab) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+	}
+
+	/** Allow adding new content to various sections of preferences UI (i.e. OPML import/export pane)
+	 * @param string $section
+	 * @return void
+	 * @see PluginHost::HOOK_PREFS_TAB_SECTION
+	 */
+	function hook_prefs_tab_section($section) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+	}
+
+	/** Allows adding new (top) tabs in preferences UI
+	 * @return void
+	 * @see PluginHost::HOOK_PREFS_TABS
+	*/
+	function hook_prefs_tabs() {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+	}
+
+	/** Invoked when feed XML is processed by FeedParser class
+	 * @param FeedParser $parser
+	 * @param int $feed_id
+	 * @return void
+	 * @see PluginHost::HOOK_FEED_PARSED
+	 */
+	function hook_feed_parsed($parser, $feed_id) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+	}
+
+	/** GLOBAL: Invoked when a feed update task finishes
+	 * @param array<string,string> $cli_options
+	 * @return void
+	 * @see PluginHost::HOOK_UPDATE_TASK
+	 */
+	function hook_update_task($cli_options) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+	}
+
+	/** This is a pluginhost compatibility wrapper that invokes $this->authenticate(...$args) (Auth_Base)
+	 * @param string $login
+	 * @param string $password
+	 * @param string $service
+	 * @return int|false user_id
+	 * @see PluginHost::HOOK_AUTH_USER
+	 */
+	function hook_auth_user($login, $password, $service = '') {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+		return false;
+	}
+
+	/** IAuthModule only
+	 * @param string $login
+	 * @param string $password
+	 * @param string $service
+	 * @return int|false user_id
+	 */
+	function authenticate($login, $password, $service = '') {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+		return false;
+	}
+
+	/** Allows plugins to modify global hotkey map (hotkey sequence -> action)
+	 * @param array<string, string> $hotkeys
+	 * @return array<string, string>
+	 * @see PluginHost::HOOK_HOTKEY_MAP
+	 * @see Plugin::hook_hotkey_info()
+	 */
+	function hook_hotkey_map($hotkeys) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return [];
+	}
+
+	/** Invoked when article is rendered by backend (before it gets passed to frontent JS code) - three panel mode
+	 * @param array<string, mixed> $article
+	 * @return array<string, mixed>
+	 * @see PluginHost::HOOK_RENDER_ARTICLE
+	 */
+	function hook_render_article($article) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return [];
+	}
+
+	/** Invoked when article is rendered by backend (before it gets passed to frontent JS code) - combined mode
+	 * @param array<string, mixed> $article
+	 * @return array<string, mixed>
+	 * @see PluginHost::HOOK_RENDER_ARTICLE_CDM
+	 */
+	function hook_render_article_cdm($article) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return [];
+	}
+
+	/** Invoked when raw feed XML data has been successfully downloaded (but not parsed yet)
+	 * @param string $feed_data
+	 * @param string $fetch_url
+	 * @param int $owner_uid
+	 * @param int $feed
+	 * @return string
+	 * @see PluginHost::HOOK_FEED_FETCHED
+	 */
+	function hook_feed_fetched($feed_data, $fetch_url, $owner_uid, $feed) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Invoked on article content when it is sanitized (i.e. potentially harmful tags removed)
+	 * @param DOMDocument $doc
+	 * @param string $site_url
+	 * @param array<string> $allowed_elements
+	 * @param array<string> $disallowed_attributes
+	 * @param int $article_id
+	 * @return DOMDocument|array<int,DOMDocument|array<string>>
+	 * @see PluginHost::HOOK_SANITIZE
+	 */
+	function hook_sanitize($doc, $site_url, $allowed_elements, $disallowed_attributes, $article_id) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return $doc;
+	}
+
+	/** Invoked when article is rendered by backend (before it gets passed to frontent JS code) - exclusive to API clients
+	 * @param array{'article': array<string,mixed>|null, 'headline': array<string,mixed>|null} $params
+	 * @return array<string, string>
+	 * @see PluginHost::HOOK_RENDER_ARTICLE_API
+	 */
+	function hook_render_article_api($params) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return [];
+	}
+
+	/** Allows adding new UI elements to tt-rss main toolbar (to the right, before Actions... dropdown)
+	 * @return string
+	 * @see PluginHost::HOOK_TOOLBAR_BUTTON
+	 */
+	function hook_toolbar_button() {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Allows adding new items to tt-rss main Actions... dropdown menu
+	 * @return string
+	 * @see PluginHost::HOOK_ACTION_ITEM
+	 */
+	function hook_action_item() {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Allows adding new UI elements to the toolbar area related to currently loaded feed headlines
+	 * @param int $feed_id
+	 * @param bool $is_cat
+	 * @return string
+	 * @see PluginHost::HOOK_HEADLINE_TOOLBAR_BUTTON
+	 */
+	function hook_headline_toolbar_button($feed_id, $is_cat) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Allows adding new hotkey action names and descriptions
+	 * @param array<string, array<string, string>> $hotkeys
+	 * @return array<string, array<string, string>>
+	 * @see PluginHost::HOOK_HOTKEY_INFO
+	 * @see Plugin::hook_hotkey_map()
+	 */
+	function hook_hotkey_info($hotkeys) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return [];
+	}
+
+	/** Adds per-article buttons on the left side
+	 * @param array<string,mixed> $row
+	 * @return string
+	 * @see PluginHost::HOOK_ARTICLE_LEFT_BUTTON
+	 * @see Plugin::hook_article_button()
+	 */
+	function hook_article_left_button($row) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Allows adding new UI elements to the "Plugins" tab of the feed editor UI
+	 * @param int $feed_id
+	 * @return void
+	 * @see PluginHost::HOOK_PREFS_EDIT_FEED
+	 */
+	function hook_prefs_edit_feed($feed_id) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+	}
+
+	/** Invoked when data is saved in the feed editor
+	 * @param int $feed_id
+	 * @return void
+	 * @see PluginHost::HOOK_PREFS_SAVE_FEED
+	 */
+	function hook_prefs_save_feed($feed_id) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+	}
+
+	/** Allows overriding built-in fetching mechanism for feeds, substituting received data if necessary
+	 * (i.e. origin site doesn't actually provide any RSS feeds), or XML is invalid
+	 * @param string $feed_data
+	 * @param string $fetch_url
+	 * @param int $owner_uid
+	 * @param int $feed
+	 * @param int $last_article_timestamp
+	 * @param string $auth_login
+	 * @param string $auth_pass
+	 * @return string (possibly mangled feed data)
+	 * @see PluginHost::HOOK_FETCH_FEED
+	 */
+	function hook_fetch_feed($feed_data, $fetch_url, $owner_uid, $feed, $last_article_timestamp, $auth_login, $auth_pass) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Invoked when headlines data ($row) has been retrieved from the database
+	 * @param array<string,mixed> $row
+	 * @param int $excerpt_length
+	 * @return array<string,mixed>
+	 * @see PluginHost::HOOK_QUERY_HEADLINES
+	 */
+	function hook_query_headlines($row, $excerpt_length) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return [];
+	}
+
+	/** GLOBAL: This is run periodically by the update daemon when idle
+	 * @return void
+	 * @see PluginHost::HOOK_HOUSE_KEEPING */
+	function hook_house_keeping() {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+	}
+
+	/** Allows overriding built-in article search
+	 * @param string $query
+	 * @return array<int, string|array<string>> - list(SQL search query, highlight keywords)
+	 * @see PluginHost::HOOK_SEARCH
+	 */
+	function hook_search($query) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return [];
+	}
+
+	/** Invoked when enclosures are rendered to HTML (when article itself is rendered)
+	 * @param string $enclosures_formatted
+	 * @param array<int, array<string, mixed>> $enclosures
+	 * @param int $article_id
+	 * @param bool $always_display_enclosures
+	 * @param string $article_content
+	 * @param bool $hide_images
+	 * @return string|array<string,array<int, array<string, mixed>>> ($enclosures_formatted, $enclosures)
+	 * @see PluginHost::HOOK_FORMAT_ENCLOSURES
+	 */
+	function hook_format_enclosures($enclosures_formatted, $enclosures, $article_id, $always_display_enclosures, $article_content, $hide_images) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Invoked during feed subscription (after data has been fetched)
+	 * @param string $contents
+	 * @param string $url
+	 * @param string $auth_login
+	 * @param string $auth_pass
+	 * @return string (possibly mangled feed data)
+	 * @see PluginHost::HOOK_SUBSCRIBE_FEED
+	 */
+	function hook_subscribe_feed($contents, $url, $auth_login, $auth_pass) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/**
+	 * @param int $feed
+	 * @param bool $is_cat
+	 * @param array<string,mixed> $qfh_ret (headlines object)
+	 * @return string
+	 * @see PluginHost::HOOK_HEADLINES_BEFORE
+	 */
+	function hook_headlines_before($feed, $is_cat, $qfh_ret) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/**
+	 * @param array<string,mixed> $entry
+	 * @param int $article_id
+	 * @param array<string,mixed> $rv
+	 * @return string
+	 * @see PluginHost::HOOK_RENDER_ENCLOSURE
+	 */
+	function hook_render_enclosure($entry, $article_id, $rv) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/**
+	 * @param array<string,mixed> $article
+	 * @param string $action
+	 * @return array<string,mixed> ($article)
+	 * @see PluginHost::HOOK_ARTICLE_FILTER_ACTION
+	 */
+	function hook_article_filter_action($article, $action) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return [];
+	}
+
+	/**
+	 * @param array<string,mixed> $line
+	 * @param int $feed
+	 * @param bool $is_cat
+	 * @param int $owner_uid
+	 * @return array<string,mixed> ($line)
+	 * @see PluginHost::HOOK_ARTICLE_EXPORT_FEED
+	 */
+	function hook_article_export_feed($line, $feed, $is_cat, $owner_uid) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return [];
+	}
+
+	/** Allows adding custom buttons to tt-rss main toolbar (left side)
+	 * @return void
+	 * @see PluginHost::HOOK_MAIN_TOOLBAR_BUTTON
+	*/
+	function hook_main_toolbar_button() {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+	}
+
+	/** Invoked for every enclosure entry as article is being rendered
+	 * @param array<string,string> $entry
+	 * @param int $id
+	 * @param array{'formatted': string, 'entries': array<int, array<string, mixed>>} $rv
+	 * @return array<string,string> ($entry)
+	 * @see PluginHost::HOOK_ENCLOSURE_ENTRY
+	 */
+	function hook_enclosure_entry($entry, $id, $rv) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return [];
+	}
+
+	/** Share plugins run this when article is being rendered as HTML for sharing
+	 * @param string $html
+	 * @param array<string,mixed> $row
+	 * @return string ($html)
+	 * @see PluginHost::HOOK_FORMAT_ARTICLE
+	 */
+	function hook_format_article($html, $row) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Invoked when basic feed information (title, site_url) is being collected, useful to override default if feed doesn't provide anything (or feed itself is synthesized)
+	 * @param array{"title": string, "site_url": string} $basic_info
+	 * @param string $fetch_url
+	 * @param int $owner_uid
+	 * @param int $feed_id
+	 * @param string $auth_login
+	 * @param string $auth_pass
+	 * @return array{"title": string, "site_url": string}
+	 * @see PluginHost::HOOK_FEED_BASIC_INFO
+	 */
+	function hook_feed_basic_info($basic_info, $fetch_url, $owner_uid, $feed_id, $auth_login, $auth_pass) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return $basic_info;
+	}
+
+	/** Invoked when file (e.g. cache entry, static data) is being sent to client, may override default mechanism
+	 * using faster httpd-specific implementation (see nginx_xaccel)
+	 * @param string $filename
+	 * @return bool
+	 * @see PluginHost::HOOK_SEND_LOCAL_FILE
+	 */
+	function hook_send_local_file($filename) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return false;
+	}
+
+	/** Invoked when user tries to unsubscribe from a feed, returning true would prevent any further default actions
+	 * @param int $feed_id
+	 * @param int $owner_uid
+	 * @return bool
+	 * @see PluginHost::HOOK_UNSUBSCRIBE_FEED
+	 */
+	function hook_unsubscribe_feed($feed_id, $owner_uid) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return false;
+	}
+
+	/** Invoked when mail is being sent (if no hooks are registered, tt-rss uses PHP mail() as a fallback)
+	 * @param Mailer $mailer
+	 * @param array<string,mixed> $params
+	 * @return int
+	 * @see PluginHost::HOOK_SEND_MAIL
+	 */
+	function hook_send_mail($mailer, $params) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return -1;
+	}
+
+	/** Invoked when filter is triggered on an article, may be used to implement logging for filters
+	 * NOTE: $article_filters should be renamed $filter_actions because that's what this is
+	 * @param int $feed_id
+	 * @param int $owner_uid
+	 * @param array<string,mixed> $article
+	 * @param array<string,mixed> $matched_filters
+	 * @param array<string,string|bool|int> $matched_rules
+	 * @param array<string,string> $article_filters
+	 * @return void
+	 * @see PluginHost::HOOK_FILTER_TRIGGERED
+	 */
+	function hook_filter_triggered($feed_id, $owner_uid, $article, $matched_filters, $matched_rules, $article_filters) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+	}
+
+	/** Plugins may provide this to allow getting full article text (af_readbility implements this)
+	 * @param string $url
+	 * @return string|false
+	 * @see PluginHost::HOOK_GET_FULL_TEXT
+	 */
+	function hook_get_full_text($url) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Invoked when article flavor image is being determined, allows overriding default selection logic
+	 * @param array<string,string> $enclosures
+	 * @param string $content
+	 * @param string $site_url
+	 * @param array<string,mixed> $article
+	 * @return string|array<int,string>
+	 * @see PluginHost::HOOK_ARTICLE_IMAGE
+	 */
+	function hook_article_image($enclosures, $content, $site_url, $article) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Allows adding arbitrary elements before feed tree
+	 * @return string HTML
+	 * @see PluginHost::HOOK_FEED_TREE
+	 * */
+	function hook_feed_tree() {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Invoked for every iframe to determine if it is allowed to be displayed
+	 * @param string $url
+	 * @return bool
+	 * @see PluginHost::HOOK_IFRAME_WHITELISTED
+	 */
+	function hook_iframe_whitelisted($url) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return false;
+	}
+
+	/**
+	 * @param object $enclosure
+	 * @param int $feed
+	 * @return object ($enclosure)
+	 * @see PluginHost::HOOK_ENCLOSURE_IMPORTED
+	 */
+	function hook_enclosure_imported($enclosure, $feed) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return $enclosure;
+	}
+
+	/** Allows adding custom elements to headline sort dropdown (name -> caption)
+	 * @return array<string,string>
+	 * @see PluginHost::HOOK_HEADLINES_CUSTOM_SORT_MAP
+	 */
+	function hook_headlines_custom_sort_map() {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return ["" => ""];
+	}
+
+	/** Allows overriding headline sorting (or provide custom sort methods)
+	 * @param string $order
+	 * @return array<int, string|bool> -- (query, skip_first_id)
+	 * @see PluginHost::HOOK_HEADLINES_CUSTOM_SORT_OVERRIDE
+	 */
+	function hook_headlines_custom_sort_override($order) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return ["", false];
+	}
+
+	/** Allows adding custom elements to headlines Select... dropdown
+	 * @param int $feed_id
+	 * @param int $is_cat
+	 * @return string
+	 * @see PluginHost::HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM
+	 */
+	function hook_headline_toolbar_select_menu_item($feed_id, $is_cat) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return "";
+	}
+
+	/** Invoked when user tries to subscribe to feed, may override information (i.e. feed URL) used afterwards
+	 * @param string $url
+	 * @param string $auth_login
+	 * @param string $auth_pass
+	 * @return bool
+	 * @see PluginHost::HOOK_PRE_SUBSCRIBE
+	 */
+	function hook_pre_subscribe(&$url, $auth_login, $auth_pass) {
+		user_error("Dummy method invoked.", E_USER_ERROR);
+
+		return false;
+	}
 }
diff --git a/classes/pluginhandler.php b/classes/pluginhandler.php
index 75b823822..5c73920e5 100644
--- a/classes/pluginhandler.php
+++ b/classes/pluginhandler.php
@@ -1,10 +1,10 @@
 <?php
 class PluginHandler extends Handler_Protected {
-	function csrf_ignore($method) {
+	function csrf_ignore(string $method): bool {
 		return true;
 	}
 
-	function catchall($method) {
+	function catchall(string $method): void {
 		$plugin_name = clean($_REQUEST["plugin"]);
 		$plugin = PluginHost::getInstance()->get_plugin($plugin_name);
 		$csrf_token = ($_POST["csrf_token"] ?? "");
diff --git a/classes/pluginhost.php b/classes/pluginhost.php
index b506a957a..f89cc5c32 100755
--- a/classes/pluginhost.php
+++ b/classes/pluginhost.php
@@ -1,186 +1,211 @@
 <?php
 class PluginHost {
-	private $pdo;
-	/* separate handle for plugin data so transaction while saving wouldn't clash with possible main
-		tt-rss code transactions; only initialized when first needed */
-	private $pdo_data;
-	private $hooks = array();
-	private $plugins = array();
-	private $handlers = array();
-	private $commands = array();
-	private $storage = array();
-	private $feeds = array();
-	private $api_methods = array();
-	private $plugin_actions = array();
-	private $owner_uid;
-	private $data_loaded;
-	private static $instance;
+	// TODO: class properties can be switched to PHP typing if/when the minimum PHP_VERSION is raised to 7.4.0+
+	/** @var PDO|null */
+	private $pdo = null;
+
+	/**
+	 * separate handle for plugin data so transaction while saving wouldn't clash with possible main
+	 * tt-rss code transactions; only initialized when first needed
+	 *
+	 * @var PDO|null
+	 */
+	private $pdo_data = null;
+
+	/** @var array<string, array<int, array<int, Plugin>>> hook types -> priority levels -> Plugins */
+	private $hooks = [];
+
+	/** @var array<string, Plugin> */
+	private $plugins = [];
+
+	/** @var array<string, array<string, Plugin>> handler type -> method type -> Plugin */
+	private $handlers = [];
+
+	/** @var array<string, array{'description': string, 'suffix': string, 'arghelp': string, 'class': Plugin}> command type -> details array */
+	private $commands = [];
+
+	/** @var array<string, array<string, mixed>> plugin name -> (potential profile array) -> key -> value  */
+	private $storage = [];
+
+	/** @var array<int, array<int, array{'id': int, 'title': string, 'sender': Plugin, 'icon': string}>> */
+	private $feeds = [];
+
+	/** @var array<string, Plugin> API method name, Plugin sender */
+	private $api_methods = [];
+
+	/** @var array<string, array<int, array{'action': string, 'description': string, 'sender': Plugin}>> */
+	private $plugin_actions = [];
+
+	/** @var int|null */
+	private $owner_uid = null;
+
+	/** @var bool */
+	private $data_loaded = false;
+
+	/** @var PluginHost|null */
+	private static $instance = null;
 
 	const API_VERSION = 2;
 	const PUBLIC_METHOD_DELIMITER = "--";
 
-	// Hooks marked with *1 are run in global context and available
-	// to plugins loaded in config.php only
-
-	/** hook_article_button($line) */
+	/** @see Plugin::hook_article_button() */
 	const HOOK_ARTICLE_BUTTON = "hook_article_button";
 
-	/** hook_article_filter($article) */
+	/** @see Plugin::hook_article_filter() */
 	const HOOK_ARTICLE_FILTER = "hook_article_filter";
 
-	/** hook_prefs_tab($tab) */
+	/** @see Plugin::hook_prefs_tab() */
 	const HOOK_PREFS_TAB = "hook_prefs_tab";
 
-	/** hook_prefs_tab_section($section) */
+	/** @see Plugin::hook_prefs_tab_section() */
 	const HOOK_PREFS_TAB_SECTION = "hook_prefs_tab_section";
 
-	/** hook_prefs_tabs() */
+	/** @see Plugin::hook_prefs_tabs() */
 	const HOOK_PREFS_TABS = "hook_prefs_tabs";
 
-	/** hook_feed_parsed($parser, $feed_id) */
+	/** @see Plugin::hook_feed_parsed() */
 	const HOOK_FEED_PARSED = "hook_feed_parsed";
 
-	/** GLOBAL: hook_update_task($cli_options) */
+	/** @see Plugin::hook_update_task() */
 	const HOOK_UPDATE_TASK = "hook_update_task"; //*1
 
-	/** hook_auth_user($login, $password, $service) (byref) */
+	/** @see Plugin::hook_auth_user() */
 	const HOOK_AUTH_USER = "hook_auth_user";
 
-	/** hook_hotkey_map($hotkeys) (byref) */
+	/** @see Plugin::hook_hotkey_map() */
 	const HOOK_HOTKEY_MAP = "hook_hotkey_map";
 
-	/** hook_render_article($article) */
+	/** @see Plugin::hook_render_article() */
 	const HOOK_RENDER_ARTICLE = "hook_render_article";
 
-	/** hook_render_article_cdm($article) */
+	/** @see Plugin::hook_render_article_cdm() */
 	const HOOK_RENDER_ARTICLE_CDM = "hook_render_article_cdm";
 
-	/** hook_feed_fetched($feed_data, $fetch_url, $owner_uid, $feed) (byref) */
+	/** @see Plugin::hook_feed_fetched() */
 	const HOOK_FEED_FETCHED = "hook_feed_fetched";
 
-	/** hook_sanitize($doc, $site_url, $allowed_elements, $disallowed_attributes, $article_id) (byref) */
+	/** @see Plugin::hook_sanitize() */
 	const HOOK_SANITIZE = "hook_sanitize";
 
-	/** hook_render_article_api($params) */
+	/** @see Plugin::hook_render_article_api() */
 	const HOOK_RENDER_ARTICLE_API = "hook_render_article_api";
 
-	/** hook_toolbar_button() */
+	/** @see Plugin::hook_toolbar_button() */
 	const HOOK_TOOLBAR_BUTTON = "hook_toolbar_button";
 
-	/** hook_action_item() */
+	/** @see Plugin::hook_action_item() */
 	const HOOK_ACTION_ITEM = "hook_action_item";
 
-	/** hook_headline_toolbar_button($feed_id, $is_cat) */
+	/** @see Plugin::hook_headline_toolbar_button() */
 	const HOOK_HEADLINE_TOOLBAR_BUTTON = "hook_headline_toolbar_button";
 
-	/** hook_hotkey_info($hotkeys) (byref) */
+	/** @see Plugin::hook_hotkey_info() */
 	const HOOK_HOTKEY_INFO = "hook_hotkey_info";
 
-	/** hook_article_left_button($row) */
+	/** @see Plugin::hook_article_left_button() */
 	const HOOK_ARTICLE_LEFT_BUTTON = "hook_article_left_button";
 
-	/** hook_prefs_edit_feed($feed_id) */
+	/** @see Plugin::hook_prefs_edit_feed() */
 	const HOOK_PREFS_EDIT_FEED = "hook_prefs_edit_feed";
 
-	/** hook_prefs_save_feed($feed_id) */
+	/** @see Plugin::hook_prefs_save_feed() */
 	const HOOK_PREFS_SAVE_FEED = "hook_prefs_save_feed";
 
-	/** hook_fetch_feed($feed_data, $fetch_url, $owner_uid, $feed, $last_article_timestamp, $auth_login, $auth_pass) (byref) */
+	/** @see Plugin::hook_fetch_feed() */
 	const HOOK_FETCH_FEED = "hook_fetch_feed";
 
-	/** hook_query_headlines($row) (byref) */
+	/** @see Plugin::hook_query_headlines() */
 	const HOOK_QUERY_HEADLINES = "hook_query_headlines";
 
-	/** GLOBAL: hook_house_keeping() */
+	/** @see Plugin::hook_house_keeping() */
 	const HOOK_HOUSE_KEEPING = "hook_house_keeping"; //*1
 
-	/** hook_search($query) */
+	/** @see Plugin::hook_search() */
 	const HOOK_SEARCH = "hook_search";
 
-	/** hook_format_enclosures($rv, $result, $id, $always_display_enclosures, $article_content, $hide_images) (byref) */
+	/** @see Plugin::hook_format_enclosures() */
 	const HOOK_FORMAT_ENCLOSURES = "hook_format_enclosures";
 
-	/** hook_subscribe_feed($contents, $url, $auth_login, $auth_pass) (byref) */
+	/** @see Plugin::hook_subscribe_feed() */
 	const HOOK_SUBSCRIBE_FEED = "hook_subscribe_feed";
 
-	/** hook_headlines_before($feed, $is_cat, $qfh_ret) */
+	/** @see Plugin::hook_headlines_before() */
 	const HOOK_HEADLINES_BEFORE = "hook_headlines_before";
 
-	/** hook_render_enclosure($entry, $id, $rv) */
+	/** @see Plugin::hook_render_enclosure() */
 	const HOOK_RENDER_ENCLOSURE = "hook_render_enclosure";
 
-	/** hook_article_filter_action($article, $action) */
+	/** @see Plugin::hook_article_filter_action() */
 	const HOOK_ARTICLE_FILTER_ACTION = "hook_article_filter_action";
 
-	/** hook_article_export_feed($line, $feed, $is_cat, $owner_uid) (byref) */
+	/** @see Plugin::hook_article_export_feed() */
 	const HOOK_ARTICLE_EXPORT_FEED = "hook_article_export_feed";
 
-	/** hook_main_toolbar_button() */
+	/** @see Plugin::hook_main_toolbar_button() */
 	const HOOK_MAIN_TOOLBAR_BUTTON = "hook_main_toolbar_button";
 
-	/** hook_enclosure_entry($entry, $id, $rv) (byref) */
+	/** @see Plugin::hook_enclosure_entry() */
 	const HOOK_ENCLOSURE_ENTRY = "hook_enclosure_entry";
 
-	/** hook_format_article($html, $row) */
+	/** @see Plugin::hook_format_article() */
 	const HOOK_FORMAT_ARTICLE = "hook_format_article";
 
-	/** @deprecated removed, do not use */
+	/** @see Plugin::hook_format_article_cdm() */
 	const HOOK_FORMAT_ARTICLE_CDM = "hook_format_article_cdm";
 
-	/** hook_feed_basic_info($basic_info, $fetch_url, $owner_uid, $feed_id, $auth_login, $auth_pass) (byref) */
+	/** @see Plugin::hook_feed_basic_info() */
 	const HOOK_FEED_BASIC_INFO = "hook_feed_basic_info";
 
-	/** hook_send_local_file($filename) */
+	/** @see Plugin::hook_send_local_file() */
 	const HOOK_SEND_LOCAL_FILE = "hook_send_local_file";
 
-	/** hook_unsubscribe_feed($feed_id, $owner_uid) */
+	/** @see Plugin::hook_unsubscribe_feed() */
 	const HOOK_UNSUBSCRIBE_FEED = "hook_unsubscribe_feed";
 
-	/** hook_send_mail(Mailer $mailer, $params) */
+	/** @see Plugin::hook_send_mail() */
 	const HOOK_SEND_MAIL = "hook_send_mail";
 
-	/** hook_filter_triggered($feed_id, $owner_uid, $article, $matched_filters, $matched_rules, $article_filters) */
+	/** @see Plugin::hook_filter_triggered() */
 	const HOOK_FILTER_TRIGGERED = "hook_filter_triggered";
 
-	/** hook_get_full_text($url) */
+	/** @see Plugin::hook_get_full_text() */
 	const HOOK_GET_FULL_TEXT = "hook_get_full_text";
 
-	/** hook_article_image($enclosures, $content, $site_url) */
+	/** @see Plugin::hook_article_image() */
 	const HOOK_ARTICLE_IMAGE = "hook_article_image";
 
-	/** hook_feed_tree() */
+	/** @see Plugin::hook_feed_tree() */
 	const HOOK_FEED_TREE = "hook_feed_tree";
 
-	/** hook_iframe_whitelisted($url) */
+	/** @see Plugin::hook_iframe_whitelisted() */
 	const HOOK_IFRAME_WHITELISTED = "hook_iframe_whitelisted";
 
-	/** hook_enclosure_imported($enclosure, $feed) */
+	/** @see Plugin::hook_enclosure_imported() */
 	const HOOK_ENCLOSURE_IMPORTED = "hook_enclosure_imported";
 
-	/** hook_headlines_custom_sort_map() */
+	/** @see Plugin::hook_headlines_custom_sort_map() */
 	const HOOK_HEADLINES_CUSTOM_SORT_MAP = "hook_headlines_custom_sort_map";
 
-	/** hook_headlines_custom_sort_override($order) */
+	/** @see Plugin::hook_headlines_custom_sort_override() */
 	const HOOK_HEADLINES_CUSTOM_SORT_OVERRIDE = "hook_headlines_custom_sort_override";
 
-	/** hook_headline_toolbar_select_menu_item($feed_id, $is_cat) */
+	/** @see Plugin::hook_headline_toolbar_select_menu_item() */
 	const HOOK_HEADLINE_TOOLBAR_SELECT_MENU_ITEM = "hook_headline_toolbar_select_menu_item";
 
-
-	/** hook_pre_subscribe($url, $auth_login, $auth_pass) (byref) */
+	/** @see Plugin::hook_pre_subscribe() */
 	const HOOK_PRE_SUBSCRIBE = "hook_pre_subscribe";
 
 	const KIND_ALL = 1;
 	const KIND_SYSTEM = 2;
 	const KIND_USER = 3;
 
-	static function object_to_domain(Plugin $plugin) {
+	static function object_to_domain(Plugin $plugin): string {
 		return strtolower(get_class($plugin));
 	}
 
 	function __construct() {
 		$this->pdo = Db::pdo();
-		$this->storage = array();
+		$this->storage = [];
 	}
 
 	private function __clone() {
@@ -194,18 +219,18 @@ class PluginHost {
 		return self::$instance;
 	}
 
-	private function register_plugin(string $name, Plugin $plugin) {
+	private function register_plugin(string $name, Plugin $plugin): void {
 		//array_push($this->plugins, $plugin);
 		$this->plugins[$name] = $plugin;
 	}
 
 	/** needed for compatibility with API 1 */
-	function get_link() {
+	function get_link(): bool {
 		return false;
 	}
 
 	/** needed for compatibility with API 2 (?) */
-	function get_dbh() {
+	function get_dbh(): bool {
 		return false;
 	}
 
@@ -213,8 +238,11 @@ class PluginHost {
 		return $this->pdo;
 	}
 
-	function get_plugin_names() {
-		$names = array();
+	/**
+	 * @return array<int, string>
+	 */
+	function get_plugin_names(): array {
+		$names = [];
 
 		foreach ($this->plugins as $p) {
 			array_push($names, get_class($p));
@@ -223,15 +251,22 @@ class PluginHost {
 		return $names;
 	}
 
-	function get_plugins() {
+	/**
+	 * @return array<Plugin>
+	 */
+	function get_plugins(): array {
 		return $this->plugins;
 	}
 
-	function get_plugin(string $name) {
+	function get_plugin(string $name): ?Plugin {
 		return $this->plugins[strtolower($name)] ?? null;
 	}
 
-	function run_hooks(string $hook, ...$args) {
+	/**
+	 * @param PluginHost::HOOK_* $hook
+	 * @param mixed $args
+	 */
+	function run_hooks(string $hook, ...$args): void {
 		$method = strtolower($hook);
 
 		foreach ($this->get_hooks($hook) as $plugin) {
@@ -247,7 +282,12 @@ class PluginHost {
 		}
 	}
 
-	function run_hooks_until(string $hook, $check, ...$args) {
+	/**
+	 * @param PluginHost::HOOK_* $hook
+	 * @param mixed $args
+	 * @param mixed $check
+	 */
+	function run_hooks_until(string $hook, $check, ...$args): bool {
 		$method = strtolower($hook);
 
 		foreach ($this->get_hooks($hook) as $plugin) {
@@ -267,7 +307,11 @@ class PluginHost {
 		return false;
 	}
 
-	function run_hooks_callback(string $hook, Closure $callback, ...$args) {
+	/**
+	 * @param PluginHost::HOOK_* $hook
+	 * @param mixed $args
+	 */
+	function run_hooks_callback(string $hook, Closure $callback, ...$args): void {
 		$method = strtolower($hook);
 
 		foreach ($this->get_hooks($hook) as $plugin) {
@@ -284,7 +328,11 @@ class PluginHost {
 		}
 	}
 
-	function chain_hooks_callback(string $hook, Closure $callback, &...$args) {
+	/**
+	 * @param PluginHost::HOOK_* $hook
+	 * @param mixed $args
+	 */
+	function chain_hooks_callback(string $hook, Closure $callback, &...$args): void {
 		$method = strtolower($hook);
 
 		foreach ($this->get_hooks($hook) as $plugin) {
@@ -301,7 +349,10 @@ class PluginHost {
 		}
 	}
 
-	function add_hook(string $type, Plugin $sender, int $priority = 50) {
+	/**
+	 * @param PluginHost::HOOK_* $type
+	 */
+	function add_hook(string $type, Plugin $sender, int $priority = 50): void {
 		$priority = (int) $priority;
 
 		if (!method_exists($sender, strtolower($type))) {
@@ -325,7 +376,10 @@ class PluginHost {
 		ksort($this->hooks[$type]);
 	}
 
-	function del_hook(string $type, Plugin $sender) {
+	/**
+	 * @param PluginHost::HOOK_* $type
+	 */
+	function del_hook(string $type, Plugin $sender): void {
 		if (is_array($this->hooks[$type])) {
 			foreach (array_keys($this->hooks[$type]) as $prio) {
 				$key = array_search($sender, $this->hooks[$type][$prio]);
@@ -337,6 +391,10 @@ class PluginHost {
 		}
 	}
 
+	/**
+	 * @param PluginHost::HOOK_* $type
+	 * @return array<int, Plugin>
+	 */
 	function get_hooks(string $type) {
 		if (isset($this->hooks[$type])) {
 			$tmp = [];
@@ -346,11 +404,14 @@ class PluginHost {
 			}
 
 			return $tmp;
-		} else {
-			return [];
 		}
+		return [];
 	}
-	function load_all(int $kind, int $owner_uid = null, bool $skip_init = false) {
+
+	/**
+	 * @param PluginHost::KIND_* $kind
+	 */
+	function load_all(int $kind, int $owner_uid = null, bool $skip_init = false): void {
 
 		$plugins = array_merge(glob("plugins/*"), glob("plugins.local/*"));
 		$plugins = array_filter($plugins, "is_dir");
@@ -361,7 +422,10 @@ class PluginHost {
 		$this->load(join(",", $plugins), $kind, $owner_uid, $skip_init);
 	}
 
-	function load(string $classlist, int $kind, int $owner_uid = null, bool $skip_init = false) {
+	/**
+	 * @param PluginHost::KIND_* $kind
+	 */
+	function load(string $classlist, int $kind, int $owner_uid = null, bool $skip_init = false): void {
 		$plugins = explode(",", $classlist);
 
 		$this->owner_uid = (int) $owner_uid;
@@ -381,8 +445,28 @@ class PluginHost {
 			}
 
 			if (!isset($this->plugins[$class])) {
+
+				// WIP hack
+				// we can't catch incompatible method signatures via Throwable
+				// this also enables global tt-rss safe mode in case there are more plugins like this
+				if (($_SESSION["plugin_blacklist"][$class] ?? 0)) {
+
+					// only report once per-plugin per-session
+					if ($_SESSION["plugin_blacklist"][$class] < 2) {
+						user_error("Plugin $class has caused a PHP fatal error so it won't be loaded again in this session.", E_USER_WARNING);
+						$_SESSION["plugin_blacklist"][$class] = 2;
+					}
+
+					$_SESSION["safe_mode"] = 1;
+
+					continue;
+				}
+
 				try {
+					$_SESSION["plugin_blacklist"][$class] = 1;
 					require_once $file;
+					unset($_SESSION["plugin_blacklist"][$class]);
+
 				} catch (Error $err) {
 					user_error($err, E_USER_WARNING);
 					continue;
@@ -434,27 +518,27 @@ class PluginHost {
 		$this->load_data();
 	}
 
-	function is_system(Plugin $plugin) {
+	function is_system(Plugin $plugin): bool {
 		$about = $plugin->about();
 
-		return $about[3] ?? false;
+		return ($about[3] ?? false) === true;
 	}
 
 	// only system plugins are allowed to modify routing
-	function add_handler(string $handler, $method, Plugin $sender) {
+	function add_handler(string $handler, string $method, Plugin $sender): void {
 		$handler = str_replace("-", "_", strtolower($handler));
 		$method = strtolower($method);
 
 		if ($this->is_system($sender)) {
 			if (!isset($this->handlers[$handler])) {
-				$this->handlers[$handler] = array();
+				$this->handlers[$handler] = [];
 			}
 
 			$this->handlers[$handler][$method] = $sender;
 		}
 	}
 
-	function del_handler(string $handler, $method, Plugin $sender) {
+	function del_handler(string $handler, string $method, Plugin $sender): void {
 		$handler = str_replace("-", "_", strtolower($handler));
 		$method = strtolower($method);
 
@@ -463,7 +547,10 @@ class PluginHost {
 		}
 	}
 
-	function lookup_handler($handler, $method) {
+	/**
+	 * @return false|Plugin false if the handler couldn't be found, otherwise the Plugin/handler
+	 */
+	function lookup_handler(string $handler, string $method) {
 		$handler = str_replace("-", "_", strtolower($handler));
 		$method = strtolower($method);
 
@@ -478,7 +565,7 @@ class PluginHost {
 		return false;
 	}
 
-	function add_command(string $command, string $description, Plugin $sender, string $suffix = "", string $arghelp = "") {
+	function add_command(string $command, string $description, Plugin $sender, string $suffix = "", string $arghelp = ""): void {
 		$command = str_replace("-", "_", strtolower($command));
 
 		$this->commands[$command] = array("description" => $description,
@@ -487,27 +574,34 @@ class PluginHost {
 			"class" => $sender);
 	}
 
-	function del_command(string $command) {
+	function del_command(string $command): void {
 		$command = "-" . strtolower($command);
 
 		unset($this->commands[$command]);
 	}
 
-	function lookup_command($command) {
+	/**
+	 * @return false|Plugin false if the command couldn't be found, otherwise the registered Plugin
+	 */
+	function lookup_command(string $command) {
 		$command = "-" . strtolower($command);
 
-		if (is_array($this->commands[$command])) {
+		if (array_key_exists($command, $this->commands) && is_array($this->commands[$command])) {
 			return $this->commands[$command]["class"];
 		} else {
 			return false;
 		}
 	}
 
+	/** @return array<string, array{'description': string, 'suffix': string, 'arghelp': string, 'class': Plugin}>> command type -> details array */
 	function get_commands() {
 		return $this->commands;
 	}
 
-	function run_commands(array $args) {
+	/**
+	 * @param array<string, mixed> $args
+	 */
+	function run_commands(array $args): void {
 		foreach ($this->get_commands() as $command => $data) {
 			if (isset($args[$command])) {
 				$command = str_replace("-", "", $command);
@@ -516,7 +610,7 @@ class PluginHost {
 		}
 	}
 
-	private function load_data() {
+	private function load_data(): void {
 		if ($this->owner_uid && !$this->data_loaded && get_schema_version() > 100)  {
 			$sth = $this->pdo->prepare("SELECT name, content FROM ttrss_plugin_storage
 				WHERE owner_uid = ?");
@@ -530,7 +624,7 @@ class PluginHost {
 		}
 	}
 
-	private function save_data(string $plugin) {
+	private function save_data(string $plugin): void {
 		if ($this->owner_uid) {
 
 			if (!$this->pdo_data)
@@ -543,7 +637,7 @@ class PluginHost {
 			$sth->execute([$this->owner_uid, $plugin]);
 
 			if (!isset($this->storage[$plugin]))
-				$this->storage[$plugin] = array();
+				$this->storage[$plugin] = [];
 
 			$content = serialize($this->storage[$plugin]);
 
@@ -563,8 +657,12 @@ class PluginHost {
 		}
 	}
 
-	// same as set(), but sets data to current preference profile
-	function profile_set(Plugin $sender, string $name, $value) {
+	/**
+	 * same as set(), but sets data to current preference profile
+	 *
+	 * @param mixed $value
+	 */
+	function profile_set(Plugin $sender, string $name, $value): void {
 		$profile_id = $_SESSION["profile"] ?? null;
 
 		if ($profile_id) {
@@ -582,26 +680,32 @@ class PluginHost {
 
 			$this->save_data(get_class($sender));
 		} else {
-			return $this->set($sender, $name, $value);
+			$this->set($sender, $name, $value);
 		}
 	}
 
-	function set(Plugin $sender, string $name, $value) {
+	/**
+	 * @param mixed $value
+	 */
+	function set(Plugin $sender, string $name, $value): void {
 		$idx = get_class($sender);
 
 		if (!isset($this->storage[$idx]))
-			$this->storage[$idx] = array();
+			$this->storage[$idx] = [];
 
 		$this->storage[$idx][$name] = $value;
 
 		$this->save_data(get_class($sender));
 	}
 
-	function set_array(Plugin $sender, array $params) {
+	/**
+	 * @param array<int|string, mixed> $params
+	 */
+	function set_array(Plugin $sender, array $params): void {
 		$idx = get_class($sender);
 
 		if (!isset($this->storage[$idx]))
-			$this->storage[$idx] = array();
+			$this->storage[$idx] = [];
 
 		foreach ($params as $name => $value)
 			$this->storage[$idx][$name] = $value;
@@ -609,7 +713,12 @@ class PluginHost {
 		$this->save_data(get_class($sender));
 	}
 
-	// same as get(), but sets data to current preference profile
+	/**
+	 * same as get(), but sets data to current preference profile
+	 *
+	 * @param mixed $default_value
+	 * @return mixed
+	 */
 	function profile_get(Plugin $sender, string $name, $default_value = false) {
 		$profile_id = $_SESSION["profile"] ?? null;
 
@@ -629,6 +738,10 @@ class PluginHost {
 		}
 	}
 
+	/**
+	 * @param mixed $default_value
+	 * @return mixed
+	 */
 	function get(Plugin $sender, string $name, $default_value = false) {
 		$idx = get_class($sender);
 
@@ -641,6 +754,10 @@ class PluginHost {
 		}
 	}
 
+	/**
+	 * @param array<int|string, mixed> $default_value
+	 * @return array<int|string, mixed>
+	 */
 	function get_array(Plugin $sender, string $name, array $default_value = []) {
 		$tmp = $this->get($sender, $name);
 
@@ -649,13 +766,16 @@ class PluginHost {
 		return $tmp;
 	}
 
-	function get_all($sender) {
+	/**
+	 * @return array<string, mixed>
+	 */
+	function get_all(Plugin $sender) {
 		$idx = get_class($sender);
 
 		return $this->storage[$idx] ?? [];
 	}
 
-	function clear_data(Plugin $sender) {
+	function clear_data(Plugin $sender): void {
 		if ($this->owner_uid) {
 			$idx = get_class($sender);
 
@@ -670,7 +790,7 @@ class PluginHost {
 	// Plugin feed functions are *EXPERIMENTAL*!
 
 	// cat_id: only -1 is supported (Special)
-	function add_feed(int $cat_id, string $title, string $icon, Plugin $sender) {
+	function add_feed(int $cat_id, string $title, string $icon, Plugin $sender): int {
 
 		if (empty($this->feeds[$cat_id]))
 			$this->feeds[$cat_id] = [];
@@ -683,12 +803,15 @@ class PluginHost {
 		return $id;
 	}
 
+	/**
+	 * @return array<int, array{'id': int, 'title': string, 'sender': Plugin, 'icon': string}>
+	 */
 	function get_feeds(int $cat_id) {
 		return $this->feeds[$cat_id] ?? [];
 	}
 
 	// convert feed_id (e.g. -129) to pfeed_id first
-	function get_feed_handler(int $pfeed_id) {
+	function get_feed_handler(int $pfeed_id): ?Plugin {
 		foreach ($this->feeds as $cat) {
 			foreach ($cat as $feed) {
 				if ($feed['id'] == $pfeed_id) {
@@ -696,46 +819,54 @@ class PluginHost {
 				}
 			}
 		}
+		return null;
 	}
 
-	static function pfeed_to_feed_id(int $pfeed) {
+	static function pfeed_to_feed_id(int $pfeed): int {
 		return PLUGIN_FEED_BASE_INDEX - 1 - abs($pfeed);
 	}
 
-	static function feed_to_pfeed_id(int $feed) {
+	static function feed_to_pfeed_id(int $feed): int {
 		return PLUGIN_FEED_BASE_INDEX - 1 + abs($feed);
 	}
 
-	function add_api_method(string $name, Plugin $sender) {
+	function add_api_method(string $name, Plugin $sender): void {
 		if ($this->is_system($sender)) {
 			$this->api_methods[strtolower($name)] = $sender;
 		}
 	}
 
-	function get_api_method(string $name) {
-		return $this->api_methods[$name];
+	function get_api_method(string $name): ?Plugin {
+		return $this->api_methods[$name] ?? null;
 	}
 
-	function add_filter_action(Plugin $sender, string $action_name, string $action_desc) {
+	function add_filter_action(Plugin $sender, string $action_name, string $action_desc): void {
 		$sender_class = get_class($sender);
 
 		if (!isset($this->plugin_actions[$sender_class]))
-			$this->plugin_actions[$sender_class] = array();
+			$this->plugin_actions[$sender_class] = [];
 
 		array_push($this->plugin_actions[$sender_class],
 			array("action" => $action_name, "description" => $action_desc, "sender" => $sender));
 	}
 
+	/**
+	 * @return array<string, array<int, array{'action': string, 'description': string, 'sender': Plugin}>>
+	 */
 	function get_filter_actions() {
 		return $this->plugin_actions;
 	}
 
-	function get_owner_uid() {
+	function get_owner_uid(): ?int {
 		return $this->owner_uid;
 	}
 
-	// handled by classes/pluginhandler.php, requires valid session
-	function get_method_url(Plugin $sender, string $method, $params = [])  {
+	/**
+	 * handled by classes/pluginhandler.php, requires valid session
+	 *
+	 * @param array<int|string, mixed> $params
+	 */
+	function get_method_url(Plugin $sender, string $method, array $params = []): string  {
 		return Config::get_self_url() . "/backend.php?" .
 			http_build_query(
 				array_merge(
@@ -758,8 +889,12 @@ class PluginHost {
 					$params));
 	} */
 
-	// WARNING: endpoint in public.php, exposed to unauthenticated users
-	function get_public_method_url(Plugin $sender, string $method, $params = [])  {
+	/**
+	 * WARNING: endpoint in public.php, exposed to unauthenticated users
+	 *
+	 * @param array<int|string, mixed> $params
+	 */
+	function get_public_method_url(Plugin $sender, string $method, array $params = []): ?string  {
 		if ($sender->is_public_method($method)) {
 			return Config::get_self_url() . "/public.php?" .
 				http_build_query(
@@ -768,18 +903,18 @@ class PluginHost {
 							"op" => strtolower(get_class($sender) . self::PUBLIC_METHOD_DELIMITER . $method),
 						],
 						$params));
-		} else {
-			user_error("get_public_method_url: requested method '$method' of '" . get_class($sender) . "' is private.");
 		}
+		user_error("get_public_method_url: requested method '$method' of '" . get_class($sender) . "' is private.");
+		return null;
 	}
 
-	function get_plugin_dir(Plugin $plugin) {
+	function get_plugin_dir(Plugin $plugin): string {
 		$ref = new ReflectionClass(get_class($plugin));
 		return dirname($ref->getFileName());
 	}
 
 	// TODO: use get_plugin_dir()
-	function is_local(Plugin $plugin) {
+	function is_local(Plugin $plugin): bool {
 		$ref = new ReflectionClass(get_class($plugin));
 		return basename(dirname(dirname($ref->getFileName()))) == "plugins.local";
 	}
diff --git a/classes/pref/feeds.php b/classes/pref/feeds.php
index ac0874259..47479e124 100755
--- a/classes/pref/feeds.php
+++ b/classes/pref/feeds.php
@@ -5,13 +5,16 @@ class Pref_Feeds extends Handler_Protected {
 	const E_ICON_UPLOAD_FAILED = 'E_ICON_UPLOAD_FAILED';
 	const E_ICON_UPLOAD_SUCCESS = 'E_ICON_UPLOAD_SUCCESS';
 
-	function csrf_ignore($method) {
+	function csrf_ignore(string $method): bool {
 		$csrf_ignored = array("index", "getfeedtree", "savefeedorder");
 
 		return array_search($method, $csrf_ignored) !== false;
 	}
 
-	public static function get_ts_languages() {
+	/**
+	 * @return array<int, string>
+	 */
+	public static function get_ts_languages(): array {
 		if (Config::get(Config::DB_TYPE) == 'pgsql') {
 			return array_map('ucfirst',
 				array_column(ORM::for_table('pg_ts_config')->select('cfgname')->find_array(), 'cfgname'));
@@ -20,7 +23,7 @@ class Pref_Feeds extends Handler_Protected {
 		return [];
 	}
 
-	function renameCat() {
+	function renameCat(): void {
 		$cat = ORM::for_table("ttrss_feed_categories")
 			->where("owner_uid", $_SESSION["uid"])
 			->find_one($_REQUEST['id']);
@@ -33,7 +36,10 @@ class Pref_Feeds extends Handler_Protected {
 		}
 	}
 
-	private function get_category_items($cat_id) {
+	/**
+	 * @return array<int, array<string, bool|int|string>>
+	 */
+	private function get_category_items(int $cat_id): array {
 
 		if (clean($_REQUEST['mode'] ?? 0) != 2)
 			$search = $_SESSION["prefs_feed_search"] ?? "";
@@ -103,11 +109,14 @@ class Pref_Feeds extends Handler_Protected {
 		return $items;
 	}
 
-	function getfeedtree() {
+	function getfeedtree(): void {
 		print json_encode($this->_makefeedtree());
 	}
 
-	function _makefeedtree() {
+	/**
+	 * @return array<string, array<int|string, mixed>|string>
+	 */
+	function _makefeedtree(): array {
 
 		if (clean($_REQUEST['mode'] ?? 0) != 2)
 			$search = $_SESSION["prefs_feed_search"] ?? "";
@@ -184,7 +193,7 @@ class Pref_Feeds extends Handler_Protected {
 			if (count($labels)) {
 				foreach ($labels as $label) {
 					$label_id = Labels::label_to_feed_id($label->id);
-					$feed = $this->feedlist_init_feed($label_id, false, 0);
+					$feed = $this->feedlist_init_feed($label_id, null, false);
 					$feed['fg_color'] = $label->fg_color;
 					$feed['bg_color'] = $label->bg_color;
 					array_push($cat['items'], $feed);
@@ -319,19 +328,22 @@ class Pref_Feeds extends Handler_Protected {
 		];
 	}
 
-	function catsortreset() {
+	function catsortreset(): void {
 		$sth = $this->pdo->prepare("UPDATE ttrss_feed_categories
 				SET order_id = 0 WHERE owner_uid = ?");
 		$sth->execute([$_SESSION['uid']]);
 	}
 
-	function feedsortreset() {
+	function feedsortreset(): void {
 		$sth = $this->pdo->prepare("UPDATE ttrss_feeds
 				SET order_id = 0 WHERE owner_uid = ?");
 		$sth->execute([$_SESSION['uid']]);
 	}
 
-	private function process_category_order(&$data_map, $item_id, $parent_id = false, $nest_level = 0) {
+	/**
+	 * @param array<string, mixed> $data_map
+	 */
+	private function process_category_order(array &$data_map, string $item_id = '', string $parent_id = '', int $nest_level = 0): void {
 
 		$prefix = "";
 		for ($i = 0; $i < $nest_level; $i++)
@@ -403,7 +415,7 @@ class Pref_Feeds extends Handler_Protected {
 		}
 	}
 
-	function savefeedorder() {
+	function savefeedorder(): void {
 		$data = json_decode($_POST['payload'], true);
 
 		#file_put_contents("/tmp/saveorder.json", clean($_POST['payload']));
@@ -417,8 +429,9 @@ class Pref_Feeds extends Handler_Protected {
 		if (is_array($data) && is_array($data['items'])) {
 #			$cat_order_id = 0;
 
+			/** @var array<int, mixed> */
 			$data_map = array();
-			$root_item = false;
+			$root_item = '';
 
 			foreach ($data['items'] as $item) {
 
@@ -439,7 +452,7 @@ class Pref_Feeds extends Handler_Protected {
 		}
 	}
 
-	function removeIcon() {
+	function removeIcon(): void {
 		$feed_id = (int) $_REQUEST["feed_id"];
 		$icon_file = Config::get(Config::ICONS_DIR) . "/$feed_id.ico";
 
@@ -459,7 +472,7 @@ class Pref_Feeds extends Handler_Protected {
 		}
 	}
 
-	function uploadIcon() {
+	function uploadIcon(): void {
 		$feed_id = (int) $_REQUEST['feed_id'];
 		$tmp_file = tempnam(Config::get(Config::CACHE_DIR) . '/upload', 'icon');
 
@@ -502,7 +515,7 @@ class Pref_Feeds extends Handler_Protected {
 		print json_encode(['rc' => $rc, 'icon_url' => Feeds::_get_icon($feed_id)]);
 	}
 
-	function editfeed() {
+	function editfeed(): void {
 		global $purge_intervals;
 		global $update_intervals;
 
@@ -564,12 +577,12 @@ class Pref_Feeds extends Handler_Protected {
 		}
 	}
 
-	private function _batch_toggle_checkbox($name) {
+	private function _batch_toggle_checkbox(string $name): string {
 		return \Controls\checkbox_tag("", false, "",
 					["data-control-for" => $name, "title" => __("Check to enable field"), "onchange" => "App.dialogOf(this).toggleField(this)"]);
 	}
 
-	function editfeeds() {
+	function editfeeds(): void {
 		global $purge_intervals;
 		global $update_intervals;
 
@@ -677,15 +690,15 @@ class Pref_Feeds extends Handler_Protected {
 		<?php
 	}
 
-	function batchEditSave() {
-		return $this->editsaveops(true);
+	function batchEditSave(): void {
+		$this->editsaveops(true);
 	}
 
-	function editSave() {
-		return $this->editsaveops(false);
+	function editSave(): void {
+		$this->editsaveops(false);
 	}
 
-	private function editsaveops($batch) {
+	private function editsaveops(bool $batch): void {
 
 		$feed_title = clean($_POST["title"]);
 		$feed_url = clean($_POST["feed_url"]);
@@ -774,11 +787,11 @@ class Pref_Feeds extends Handler_Protected {
 						break;
 
 					case "update_interval":
-						$qpart = "update_interval = " . $this->pdo->quote($upd_intl);
+						$qpart = "update_interval = " . $upd_intl; // made int above
 						break;
 
 					case "purge_interval":
-						$qpart = "purge_interval =" . $this->pdo->quote($purge_intl);
+						$qpart = "purge_interval = " . $purge_intl; // made int above
 						break;
 
 					case "auth_login":
@@ -790,33 +803,33 @@ class Pref_Feeds extends Handler_Protected {
 						break;
 
 					case "private":
-						$qpart = "private = " . $this->pdo->quote($private);
+						$qpart = "private = " . $private; // made int above
 						break;
 
 					case "include_in_digest":
-						$qpart = "include_in_digest = " . $this->pdo->quote($include_in_digest);
+						$qpart = "include_in_digest = " . $include_in_digest; // made int above
 						break;
 
 					case "always_display_enclosures":
-						$qpart = "always_display_enclosures = " . $this->pdo->quote($always_display_enclosures);
+						$qpart = "always_display_enclosures = " . $always_display_enclosures; // made int above
 						break;
 
 					case "mark_unread_on_update":
-						$qpart = "mark_unread_on_update = " . $this->pdo->quote($mark_unread_on_update);
+						$qpart = "mark_unread_on_update = " . $mark_unread_on_update; // made int above
 						break;
 
 					case "cache_images":
-						$qpart = "cache_images = " . $this->pdo->quote($cache_images);
+						$qpart = "cache_images = " . $cache_images; // made int above
 						break;
 
 					case "hide_images":
-						$qpart = "hide_images = " . $this->pdo->quote($hide_images);
+						$qpart = "hide_images = " . $hide_images; // made int above
 						break;
 
 					case "cat_id":
 						if (get_pref(Prefs::ENABLE_FEED_CATS)) {
 							if ($cat_id) {
-								$qpart = "cat_id = " . $this->pdo->quote($cat_id);
+								$qpart = "cat_id = " . $cat_id; // made int above
 							} else {
 								$qpart = 'cat_id = NULL';
 							}
@@ -841,39 +854,36 @@ class Pref_Feeds extends Handler_Protected {
 
 			$this->pdo->commit();
 		}
-		return;
 	}
 
-	function remove() {
-
-		$ids = explode(",", clean($_REQUEST["ids"]));
+	function remove(): void {
+		/** @var array<int, int> */
+		$ids = array_map('intval', explode(",", clean($_REQUEST["ids"])));
 
 		foreach ($ids as $id) {
 			self::remove_feed($id, $_SESSION["uid"]);
 		}
-
-		return;
 	}
 
-	function removeCat() {
+	function removeCat(): void {
 		$ids = explode(",", clean($_REQUEST["ids"]));
 		foreach ($ids as $id) {
 			Feeds::_remove_cat((int)$id, $_SESSION["uid"]);
 		}
 	}
 
-	function addCat() {
+	function addCat(): void {
 		$feed_cat = clean($_REQUEST["cat"]);
 
 		Feeds::_add_cat($feed_cat, $_SESSION['uid']);
 	}
 
-	function importOpml() {
+	function importOpml(): void {
 		$opml = new OPML($_REQUEST);
 		$opml->opml_import($_SESSION["uid"]);
 	}
 
-	private function index_feeds() {
+	private function index_feeds(): void {
 		$error_button = "<button dojoType='dijit.form.Button'
 				id='pref_feeds_errors_btn' style='display : none'
 				onclick='CommonDialogs.showFeedsWithErrors()'>".
@@ -984,7 +994,7 @@ class Pref_Feeds extends Handler_Protected {
 
 	}
 
-	private function index_opml() {
+	private function index_opml(): void {
 		?>
 
 		<form id='opml_import_form' method='post' enctype='multipart/form-data'>
@@ -1020,7 +1030,7 @@ class Pref_Feeds extends Handler_Protected {
 		PluginHost::getInstance()->run_hooks(PluginHost::HOOK_PREFS_TAB_SECTION, "prefFeedsOPML");
 	}
 
-	private function index_shared() {
+	private function index_shared(): void {
 		?>
 
 		<?= format_notice('Published articles can be subscribed by anyone who knows the following URL:') ?></h3>
@@ -1040,7 +1050,7 @@ class Pref_Feeds extends Handler_Protected {
 		PluginHost::getInstance()->run_hooks(PluginHost::HOOK_PREFS_TAB_SECTION, "prefFeedsPublishedGenerated");
 	}
 
-	function index() {
+	function index(): void {
 		?>
 
 		<div dojoType='dijit.layout.TabContainer' tabPosition='left-h'>
@@ -1079,44 +1089,44 @@ class Pref_Feeds extends Handler_Protected {
 		<?php
 	}
 
-	private function feedlist_init_cat($cat_id) {
-		$obj = array();
-		$cat_id = (int) $cat_id;
-
-		$obj['id'] = 'CAT:' . $cat_id;
-		$obj['items'] = array();
-		$obj['name'] = Feeds::_get_cat_title($cat_id);
-		$obj['type'] = 'category';
-		$obj['unread'] = -1; //(int) Feeds::_get_cat_unread($cat_id);
-		$obj['bare_id'] = $cat_id;
-
-		return $obj;
+	/**
+	 * @return array<string, mixed>
+	 */
+	private function feedlist_init_cat(int $cat_id): array {
+		return [
+			'id' => 'CAT:' . $cat_id,
+			'items' => array(),
+			'name' => Feeds::_get_cat_title($cat_id),
+			'type' => 'category',
+			'unread' => -1, //(int) Feeds::_get_cat_unread($cat_id);
+			'bare_id' => $cat_id,
+		];
 	}
 
-	private function feedlist_init_feed($feed_id, $title = false, $unread = false, $error = '', $updated = '') {
-		$obj = array();
-		$feed_id = (int) $feed_id;
-
+	/**
+	 * @return array<string, mixed>
+	 */
+	private function feedlist_init_feed(int $feed_id, ?string $title = null, bool $unread = false, string $error = '', string $updated = ''): array {
 		if (!$title)
 			$title = Feeds::_get_title($feed_id, false);
 
 		if ($unread === false)
 			$unread = getFeedUnread($feed_id, false);
 
-		$obj['id'] = 'FEED:' . $feed_id;
-		$obj['name'] = $title;
-		$obj['unread'] = (int) $unread;
-		$obj['type'] = 'feed';
-		$obj['error'] = $error;
-		$obj['updated'] = $updated;
-		$obj['icon'] = Feeds::_get_icon($feed_id);
-		$obj['bare_id'] = $feed_id;
-		$obj['auxcounter'] = 0;
-
-		return $obj;
+		return [
+			'id' => 'FEED:' . $feed_id,
+			'name' => $title,
+			'unread' => (int) $unread,
+			'type' => 'feed',
+			'error' => $error,
+			'updated' => $updated,
+			'icon' => Feeds::_get_icon($feed_id),
+			'bare_id' => $feed_id,
+			'auxcounter' => 0,
+		];
 	}
 
-	function inactiveFeeds() {
+	function inactiveFeeds(): void {
 
 		if (Config::get(Config::DB_TYPE) == "pgsql") {
 			$interval_qpart = "NOW() - INTERVAL '3 months'";
@@ -1150,7 +1160,7 @@ class Pref_Feeds extends Handler_Protected {
 		print json_encode($inactive_feeds);
 	}
 
-	function feedsWithErrors() {
+	function feedsWithErrors(): void {
 		print json_encode(ORM::for_table('ttrss_feeds')
 			->select_many('id', 'title', 'feed_url', 'last_error', 'site_url')
 			->where_not_equal('last_error', '')
@@ -1158,7 +1168,7 @@ class Pref_Feeds extends Handler_Protected {
 			->find_array());
 	}
 
-	static function remove_feed($id, $owner_uid) {
+	static function remove_feed(int $id, int $owner_uid): void {
 
 		if (PluginHost::getInstance()->run_hooks_until(PluginHost::HOOK_UNSUBSCRIBE_FEED, true, $id, $owner_uid))
 			return;
@@ -1199,14 +1209,14 @@ class Pref_Feeds extends Handler_Protected {
 		}
 	}
 
-	function batchSubscribe() {
+	function batchSubscribe(): void {
 		print json_encode([
 			"enable_cats" => (int)get_pref(Prefs::ENABLE_FEED_CATS),
 			"cat_select" => \Controls\select_feeds_cats("cat")
 		]);
 	}
 
-	function batchAddFeeds() {
+	function batchAddFeeds(): void {
 		$cat_id = clean($_REQUEST['cat']);
 		$feeds = explode("\n", clean($_REQUEST['feeds']));
 		$login = clean($_REQUEST['login']);
@@ -1216,7 +1226,7 @@ class Pref_Feeds extends Handler_Protected {
 
 		// TODO: we should return some kind of error code to frontend here
 		if ($user->access_level == UserHelper::ACCESS_LEVEL_READONLY) {
-			return false;
+			return;
 		}
 
 		$csth = $this->pdo->prepare("SELECT id FROM ttrss_feeds
@@ -1244,11 +1254,11 @@ class Pref_Feeds extends Handler_Protected {
 		}
 	}
 
-	function clearKeys() {
-		return Feeds::_clear_access_keys($_SESSION['uid']);
+	function clearKeys(): void {
+		Feeds::_clear_access_keys($_SESSION['uid']);
 	}
 
-	function regenFeedKey() {
+	function regenFeedKey(): void {
 		$feed_id = clean($_REQUEST['id']);
 		$is_cat = clean($_REQUEST['is_cat']);
 
@@ -1257,7 +1267,7 @@ class Pref_Feeds extends Handler_Protected {
 		print json_encode(["link" => $new_key]);
 	}
 
-	function getSharedURL() {
+	function getSharedURL(): void {
 		$feed_id = clean($_REQUEST['id']);
 		$is_cat = clean($_REQUEST['is_cat']) == "true";
 		$search = clean($_REQUEST['search']);
@@ -1276,7 +1286,10 @@ class Pref_Feeds extends Handler_Protected {
 		]);
 	}
 
-	private function calculate_children_count($cat) {
+	/**
+	 * @param array<string, mixed> $cat
+	 */
+	private function calculate_children_count(array $cat): int {
 		$c = 0;
 
 		foreach ($cat['items'] ?? [] as $child) {
diff --git a/classes/pref/filters.php b/classes/pref/filters.php
index c4017e4ec..6e6e3d9ee 100755
--- a/classes/pref/filters.php
+++ b/classes/pref/filters.php
@@ -1,20 +1,19 @@
 <?php
 class Pref_Filters extends Handler_Protected {
 
-	function csrf_ignore($method) {
+	function csrf_ignore(string $method): bool {
 		$csrf_ignored = array("index", "getfiltertree", "savefilterorder");
 
 		return array_search($method, $csrf_ignored) !== false;
 	}
 
-	function filtersortreset() {
+	function filtersortreset(): void {
 		$sth = $this->pdo->prepare("UPDATE ttrss_filters2
 				SET order_id = 0 WHERE owner_uid = ?");
 		$sth->execute([$_SESSION['uid']]);
-		return;
 	}
 
-	function savefilterorder() {
+	function savefilterorder(): void {
 		$data = json_decode($_POST['payload'], true);
 
 		#file_put_contents("/tmp/saveorder.json", clean($_POST['payload']));
@@ -40,11 +39,9 @@ class Pref_Filters extends Handler_Protected {
 				}
 			}
 		}
-
-		return;
 	}
 
-	function testFilterDo() {
+	function testFilterDo(): void {
 		$offset = (int) clean($_REQUEST["offset"]);
 		$limit = (int) clean($_REQUEST["limit"]);
 
@@ -59,7 +56,9 @@ class Pref_Filters extends Handler_Protected {
 
 		$res = $this->pdo->query("SELECT id,name FROM ttrss_filter_types");
 
-		$filter_types = array();
+		/** @var array<int, string> */
+		$filter_types = [];
+
 		while ($line = $res->fetch()) {
 			$filter_types[$line["id"]] = $line["name"];
 		}
@@ -67,7 +66,10 @@ class Pref_Filters extends Handler_Protected {
 		$scope_qparts = array();
 
 		$rctr = 0;
+
+		/** @var string $r */
 		foreach (clean($_REQUEST["rule"]) AS $r) {
+			/** @var array{'reg_exp': string, 'filter_type': int, 'feed_id': array<int, int|string>, 'name': string}|null */
 			$rule = json_decode($r, true);
 
 			if ($rule && $rctr < 5) {
@@ -75,19 +77,21 @@ class Pref_Filters extends Handler_Protected {
 				unset($rule["filter_type"]);
 
 				$scope_inner_qparts = [];
+
+				/** @var int|string $feed_id may be a category string (e.g. 'CAT:7') or feed ID int */
 				foreach ($rule["feed_id"] as $feed_id) {
 
-                    if (strpos($feed_id, "CAT:") === 0) {
-                        $cat_id = (int) substr($feed_id, 4);
-                        array_push($scope_inner_qparts, "cat_id = " . $this->pdo->quote($cat_id));
-                    } else if ($feed_id > 0) {
-                        array_push($scope_inner_qparts, "feed_id = " . $this->pdo->quote($feed_id));
-                    }
-                }
+				if (strpos("$feed_id", "CAT:") === 0) {
+						$cat_id = (int) substr("$feed_id", 4);
+						array_push($scope_inner_qparts, "cat_id = " . $cat_id);
+					} else if (is_numeric($feed_id) && $feed_id > 0) {
+						array_push($scope_inner_qparts, "feed_id = " . (int)$feed_id);
+					}
+				}
 
-                if (count($scope_inner_qparts) > 0) {
-				    array_push($scope_qparts, "(" . implode(" OR ", $scope_inner_qparts) . ")");
-                }
+				if (count($scope_inner_qparts) > 0) {
+				array_push($scope_qparts, "(" . implode(" OR ", $scope_inner_qparts) . ")");
+				}
 
 				array_push($filter["rules"], $rule);
 
@@ -162,7 +166,7 @@ class Pref_Filters extends Handler_Protected {
 		print json_encode($rv);
 	}
 
-	private function _get_rules_list($filter_id) {
+	private function _get_rules_list(int $filter_id): string {
 		$sth = $this->pdo->prepare("SELECT reg_exp,
 			inverse,
 			match_on,
@@ -222,7 +226,7 @@ class Pref_Filters extends Handler_Protected {
 		return $rv;
 	}
 
-	function getfiltertree() {
+	function getfiltertree(): void {
 		$root = array();
 		$root['id'] = 'root';
 		$root['name'] = __('Filters');
@@ -307,10 +311,9 @@ class Pref_Filters extends Handler_Protected {
 		$fl['items'] = array($root);
 
 		print json_encode($fl);
-		return;
 	}
 
-	function edit() {
+	function edit(): void {
 
 		$filter_id = (int) clean($_REQUEST["id"] ?? 0);
 
@@ -406,7 +409,10 @@ class Pref_Filters extends Handler_Protected {
 		}
 	}
 
-	private function _get_rule_name($rule) {
+	/**
+	 * @param array<string, mixed>|null $rule
+	 */
+	private function _get_rule_name(?array $rule = null): string {
 		if (!$rule) $rule = json_decode(clean($_REQUEST["rule"]), true);
 
 		$feeds = $rule["feed_id"];
@@ -446,11 +452,18 @@ class Pref_Filters extends Handler_Protected {
 			"<span class='field'>$filter_type</span>", "<span class='feed'>$feed</span>", isset($rule["inverse"]) ? __("(inverse)") : "") . "</span>";
 	}
 
-	function printRuleName() {
+	function printRuleName(): void {
 		print $this->_get_rule_name(json_decode(clean($_REQUEST["rule"]), true));
 	}
 
-	private function _get_action_name($action) {
+	/**
+	 * @param array<string, mixed>|null $action
+	 */
+	private function _get_action_name(?array $action = null): string {
+		if (!$action) {
+			return "";
+		}
+
 		$sth = $this->pdo->prepare("SELECT description FROM
 			ttrss_filter_actions WHERE id = ?");
 		$sth->execute([(int)$action["action_id"]]);
@@ -484,12 +497,12 @@ class Pref_Filters extends Handler_Protected {
 		return $title;
 	}
 
-	function printActionName() {
-		print $this->_get_action_name(json_decode(clean($_REQUEST["action"]), true));
+	function printActionName(): void {
+		print $this->_get_action_name(json_decode(clean($_REQUEST["action"] ?? ""), true));
 	}
 
-	function editSave() {
-		$filter_id = clean($_REQUEST["id"]);
+	function editSave(): void {
+		$filter_id = (int) clean($_REQUEST["id"]);
 		$enabled = checkbox_to_sql_bool(clean($_REQUEST["enabled"] ?? false));
 		$match_any_rule = checkbox_to_sql_bool(clean($_REQUEST["match_any_rule"] ?? false));
 		$inverse = checkbox_to_sql_bool(clean($_REQUEST["inverse"] ?? false));
@@ -510,7 +523,7 @@ class Pref_Filters extends Handler_Protected {
 		$this->pdo->commit();
 	}
 
-	function remove() {
+	function remove(): void {
 
 		$ids = explode(",", clean($_REQUEST["ids"]));
 		$ids_qmarks = arr_qmarks($ids);
@@ -520,7 +533,7 @@ class Pref_Filters extends Handler_Protected {
 		$sth->execute(array_merge($ids, [$_SESSION['uid']]));
 	}
 
-	private function _save_rules_and_actions($filter_id) {
+	private function _save_rules_and_actions(int $filter_id): void {
 
 		$sth = $this->pdo->prepare("DELETE FROM ttrss_filters2_rules WHERE filter_id = ?");
 		$sth->execute([$filter_id]);
@@ -597,7 +610,7 @@ class Pref_Filters extends Handler_Protected {
 		}
 	}
 
-	function add () {
+	function add(): void {
 		$enabled = checkbox_to_sql_bool(clean($_REQUEST["enabled"] ?? false));
 		$match_any_rule = checkbox_to_sql_bool(clean($_REQUEST["match_any_rule"] ?? false));
 		$title = clean($_REQUEST["title"]);
@@ -625,7 +638,7 @@ class Pref_Filters extends Handler_Protected {
 		$this->pdo->commit();
 	}
 
-	function index() {
+	function index(): void {
 		if (array_key_exists("search", $_REQUEST)) {
 			$filter_search = clean($_REQUEST["search"]);
 			$_SESSION["prefs_filter_search"] = $filter_search;
@@ -691,7 +704,8 @@ class Pref_Filters extends Handler_Protected {
 		<?php
 	}
 
-	function editrule() {
+	function editrule(): void {
+		/** @var array<int, int|string> */
 		$feed_ids = explode(",", clean($_REQUEST["ids"]));
 
 		print json_encode([
@@ -699,7 +713,10 @@ class Pref_Filters extends Handler_Protected {
 		]);
 	}
 
-	private function _get_name($id) {
+	/**
+	 * @return array<int, string>
+	 */
+	private function _get_name(int $id): array {
 
 		$sth = $this->pdo->prepare(
 			"SELECT title,match_any_rule,f.inverse AS inverse,COUNT(DISTINCT r.id) AS num_rules,COUNT(DISTINCT a.id) AS num_actions
@@ -745,8 +762,9 @@ class Pref_Filters extends Handler_Protected {
 		return [];
 	}
 
-	function join() {
-		$ids = explode(",", clean($_REQUEST["ids"]));
+	function join(): void {
+		/** @var array<int, int> */
+		$ids = array_map("intval", explode(",", clean($_REQUEST["ids"])));
 
 		if (count($ids) > 1) {
 			$base_id = array_shift($ids);
@@ -775,7 +793,7 @@ class Pref_Filters extends Handler_Protected {
 		}
 	}
 
-	private function _optimize($id) {
+	private function _optimize(int $id): void {
 
 		$this->pdo->beginTransaction();
 
@@ -830,9 +848,11 @@ class Pref_Filters extends Handler_Protected {
 		$this->pdo->commit();
 	}
 
-	private function _feed_multi_select($id, $default_ids = [],
-						   $attributes = "", $include_all_feeds = true,
-						   $root_id = null, $nest_level = 0) {
+	/**
+	 * @param array<int, int|string> $default_ids
+	 */
+	private function _feed_multi_select(string $id, array $default_ids = [], string $attributes = "",
+		bool $include_all_feeds = true, ?int $root_id = null, int $nest_level = 0): string {
 
 		$pdo = Db::pdo();
 
diff --git a/classes/pref/labels.php b/classes/pref/labels.php
index 0eb88ea36..a50a85a66 100644
--- a/classes/pref/labels.php
+++ b/classes/pref/labels.php
@@ -1,13 +1,13 @@
 <?php
 class Pref_Labels extends Handler_Protected {
 
-	function csrf_ignore($method) {
+	function csrf_ignore(string $method): bool {
 		$csrf_ignored = array("index", "getlabeltree");
 
 		return array_search($method, $csrf_ignored) !== false;
 	}
 
-	function edit() {
+	function edit(): void {
 		$label = ORM::for_table('ttrss_labels2')
 			->where('owner_uid', $_SESSION['uid'])
 			->find_one($_REQUEST['id']);
@@ -17,7 +17,7 @@ class Pref_Labels extends Handler_Protected {
 		}
 	}
 
-	function getlabeltree() {
+	function getlabeltree(): void {
 		$root = array();
 		$root['id'] = 'root';
 		$root['name'] = __('Labels');
@@ -48,10 +48,9 @@ class Pref_Labels extends Handler_Protected {
 		$fl['items'] = array($root);
 
 		print json_encode($fl);
-		return;
 	}
 
-	function colorset() {
+	function colorset(): void {
 		$kind = clean($_REQUEST["kind"]);
 		$ids = explode(',', clean($_REQUEST["ids"]));
 		$color = clean($_REQUEST["color"]);
@@ -84,7 +83,7 @@ class Pref_Labels extends Handler_Protected {
 		}
 	}
 
-	function colorreset() {
+	function colorreset(): void {
 		$ids = explode(',', clean($_REQUEST["ids"]));
 
 		foreach ($ids as $id) {
@@ -101,7 +100,7 @@ class Pref_Labels extends Handler_Protected {
 		}
 	}
 
-	function save() {
+	function save(): void {
 
 		$id = clean($_REQUEST["id"]);
 		$caption = clean($_REQUEST["caption"]);
@@ -148,9 +147,9 @@ class Pref_Labels extends Handler_Protected {
 
 	}
 
-	function remove() {
-
-		$ids = explode(",", clean($_REQUEST["ids"]));
+	function remove(): void {
+		/** @var array<int, int> */
+		$ids = array_map("intval", explode(",", clean($_REQUEST["ids"])));
 
 		foreach ($ids as $id) {
 			Labels::remove($id, $_SESSION["uid"]);
@@ -158,7 +157,7 @@ class Pref_Labels extends Handler_Protected {
 
 	}
 
-	function add() {
+	function add(): void {
 		$caption = clean($_REQUEST["caption"]);
 		$output = clean($_REQUEST["output"] ?? false);
 
@@ -171,7 +170,7 @@ class Pref_Labels extends Handler_Protected {
 		}
 	}
 
-	function index() {
+	function index(): void {
 		?>
 		<div dojoType='dijit.layout.BorderContainer' gutters='false'>
 			<div style='padding : 0px' dojoType='dijit.layout.ContentPane' region='top'>
diff --git a/classes/pref/prefs.php b/classes/pref/prefs.php
index c45d6d6ea..d3a5a1370 100644
--- a/classes/pref/prefs.php
+++ b/classes/pref/prefs.php
@@ -2,12 +2,21 @@
 use chillerlan\QRCode;
 
 class Pref_Prefs extends Handler_Protected {
-
+	// TODO: class properties can be switched to PHP typing if/when the minimum PHP_VERSION is raised to 7.4.0+
+	/** @var array<Prefs::*, array<int, string>> */
 	private $pref_help = [];
+
+	/** @var array<string, array<int, string>> pref items are Prefs::*|Pref_Prefs::BLOCK_SEPARATOR (PHPStan was complaining) */
 	private $pref_item_map = [];
+
+	/** @var array<string, string> */
 	private $pref_help_bottom = [];
+
+	/** @var array<int, string> */
 	private $pref_blacklist = [];
 
+	private const BLOCK_SEPARATOR = 'BLOCK_SEPARATOR';
+
 	const PI_RES_ALREADY_INSTALLED = "PI_RES_ALREADY_INSTALLED";
 	const PI_RES_SUCCESS = "PI_RES_SUCCESS";
 	const PI_ERR_NO_CLASS = "PI_ERR_NO_CLASS";
@@ -17,7 +26,8 @@ class Pref_Prefs extends Handler_Protected {
 	const PI_ERR_PLUGIN_NOT_FOUND = "PI_ERR_PLUGIN_NOT_FOUND";
 	const PI_ERR_NO_WORKDIR = "PI_ERR_NO_WORKDIR";
 
-	function csrf_ignore($method) {
+	/** @param string $method */
+	function csrf_ignore($method) : bool {
 		$csrf_ignored = array("index", "updateself", "otpqrcode");
 
 		return array_search($method, $csrf_ignored) !== false;
@@ -30,35 +40,35 @@ class Pref_Prefs extends Handler_Protected {
 			__('General') => [
 				Prefs::USER_LANGUAGE,
 				Prefs::USER_TIMEZONE,
-				'BLOCK_SEPARATOR',
+				self::BLOCK_SEPARATOR,
 				Prefs::USER_CSS_THEME,
-				'BLOCK_SEPARATOR',
+				self::BLOCK_SEPARATOR,
 				Prefs::ENABLE_API_ACCESS,
 			],
 			__('Feeds') => [
 				Prefs::DEFAULT_UPDATE_INTERVAL,
 				Prefs::FRESH_ARTICLE_MAX_AGE,
 				Prefs::DEFAULT_SEARCH_LANGUAGE,
-				'BLOCK_SEPARATOR',
+				self::BLOCK_SEPARATOR,
 				Prefs::ENABLE_FEED_CATS,
-				'BLOCK_SEPARATOR',
+				self::BLOCK_SEPARATOR,
 				Prefs::CONFIRM_FEED_CATCHUP,
 				Prefs::ON_CATCHUP_SHOW_NEXT_FEED,
-				'BLOCK_SEPARATOR',
+				self::BLOCK_SEPARATOR,
 				Prefs::HIDE_READ_FEEDS,
 				Prefs::HIDE_READ_SHOWS_SPECIAL,
 			],
 			__('Articles') => [
 				Prefs::PURGE_OLD_DAYS,
 				Prefs::PURGE_UNREAD_ARTICLES,
-				'BLOCK_SEPARATOR',
+				self::BLOCK_SEPARATOR,
 				Prefs::COMBINED_DISPLAY_MODE,
 				Prefs::CDM_EXPANDED,
 				Prefs::CDM_ENABLE_GRID,
-				'BLOCK_SEPARATOR',
+				self::BLOCK_SEPARATOR,
 				Prefs::CDM_AUTO_CATCHUP,
 				Prefs::VFEED_GROUP_BY_FEED,
-				'BLOCK_SEPARATOR',
+				self::BLOCK_SEPARATOR,
 				Prefs::SHOW_CONTENT_PREVIEW,
 				Prefs::STRIP_IMAGES,
 			],
@@ -69,12 +79,12 @@ class Pref_Prefs extends Handler_Protected {
 			],
 			__('Advanced') => [
 				Prefs::BLACKLISTED_TAGS,
-				'BLOCK_SEPARATOR',
+				self::BLOCK_SEPARATOR,
 				Prefs::LONG_DATE_FORMAT,
 				Prefs::SHORT_DATE_FORMAT,
-				'BLOCK_SEPARATOR',
+				self::BLOCK_SEPARATOR,
 				Prefs::SSL_CERT_SERIAL,
-				'BLOCK_SEPARATOR',
+				self::BLOCK_SEPARATOR,
 				Prefs::DISABLE_CONDITIONAL_COUNTERS,
 				Prefs::HEADLINES_NO_DISTINCT,
 			],
@@ -127,7 +137,7 @@ class Pref_Prefs extends Handler_Protected {
 		];
 	}
 
-	function changepassword() {
+	function changepassword(): void {
 
 		if (Config::get(Config::FORBID_PASSWORD_CHANGES)) {
 			print "ERROR: ".format_error("Access forbidden.");
@@ -173,7 +183,7 @@ class Pref_Prefs extends Handler_Protected {
 		}
 	}
 
-	function saveconfig() {
+	function saveconfig(): void {
 		$boolean_prefs = explode(",", clean($_POST["boolean_prefs"]));
 
 		foreach ($boolean_prefs as $pref) {
@@ -223,7 +233,7 @@ class Pref_Prefs extends Handler_Protected {
 		}
 	}
 
-	function changePersonalData() {
+	function changePersonalData(): void {
 
 		$user = ORM::for_table('ttrss_users')->find_one($_SESSION['uid']);
 		$new_email = clean($_POST['email']);
@@ -264,13 +274,13 @@ class Pref_Prefs extends Handler_Protected {
 		print __("Your personal data has been saved.");
 	}
 
-	function resetconfig() {
+	function resetconfig(): void {
 		Prefs::reset($_SESSION["uid"], $_SESSION["profile"]);
 
 		print "PREFS_NEED_RELOAD";
 	}
 
-	private function index_auth_personal() {
+	private function index_auth_personal(): void {
 
 		$user = ORM::for_table('ttrss_users')->find_one($_SESSION['uid']);
 
@@ -310,7 +320,7 @@ class Pref_Prefs extends Handler_Protected {
 		<?php
 	}
 
-	private function index_auth_password() {
+	private function index_auth_password(): void {
 		if ($_SESSION["auth_module"]) {
 			$authenticator = PluginHost::getInstance()->get_plugin($_SESSION["auth_module"]);
 		} else {
@@ -385,7 +395,7 @@ class Pref_Prefs extends Handler_Protected {
 		}
 	}
 
-	private function index_auth_app_passwords() {
+	private function index_auth_app_passwords(): void {
 		print_notice("Separate passwords used for API clients. Required if you enable OTP.");
 		?>
 
@@ -409,7 +419,7 @@ class Pref_Prefs extends Handler_Protected {
 		<?php
 	}
 
-	private function index_auth_2fa() {
+	private function index_auth_2fa(): void {
 		$otp_enabled = UserHelper::is_otp_enabled($_SESSION["uid"]);
 
 		if ($_SESSION["auth_module"] == "auth_internal") {
@@ -515,7 +525,7 @@ class Pref_Prefs extends Handler_Protected {
 		}
 	}
 
-	function index_auth() {
+	function index_auth(): void {
 		?>
 		<div dojoType='dijit.layout.TabContainer'>
 			<div dojoType='dijit.layout.ContentPane' title="<?= __('Personal data') ?>">
@@ -534,35 +544,38 @@ class Pref_Prefs extends Handler_Protected {
 		<?php
 	}
 
-	private function index_prefs_list() {
+	private function index_prefs_list(): void {
 		$profile = $_SESSION["profile"] ?? null;
 
 		if ($profile) {
 			print_notice(__("Some preferences are only available in default profile."));
 		}
 
+		/** @var array<string, array{'type_hint': Config::T_*, 'value': bool|int|string, 'help_text': string, 'short_desc': string}> */
 		$prefs_available = [];
+
+		/** @var array<int, string> */
 		$listed_boolean_prefs = [];
 
-		foreach (Prefs::get_all($_SESSION["uid"], $profile) as $line) {
+		foreach (Prefs::get_all($_SESSION["uid"], $profile) as $pref) {
 
-			if (in_array($line["pref_name"], $this->pref_blacklist)) {
+			if (in_array($pref["pref_name"], $this->pref_blacklist)) {
 				continue;
 			}
 
-			if ($profile && in_array($line["pref_name"], Prefs::_PROFILE_BLACKLIST)) {
+			if ($profile && in_array($pref["pref_name"], Prefs::_PROFILE_BLACKLIST)) {
 				continue;
 			}
 
-			$pref_name = $line["pref_name"];
+			$pref_name = $pref["pref_name"];
 			$short_desc = $this->_get_short_desc($pref_name);
 
 			if (!$short_desc)
 				continue;
 
 			$prefs_available[$pref_name] = [
-				'type_hint' => $line['type_hint'],
-				'value' => $line['value'],
+				'type_hint' => $pref['type_hint'],
+				'value' => $pref['value'],
 				'help_text' => $this->_get_help_text($pref_name),
 				'short_desc' => $short_desc
 			];
@@ -574,12 +587,12 @@ class Pref_Prefs extends Handler_Protected {
 
 			foreach ($this->pref_item_map[$section] as $pref_name) {
 
-				if ($pref_name == 'BLOCK_SEPARATOR' && !$profile) {
+				if ($pref_name == self::BLOCK_SEPARATOR && !$profile) {
 					print "<hr/>";
 					continue;
 				}
 
-				if ($pref_name == "DEFAULT_SEARCH_LANGUAGE" && Config::get(Config::DB_TYPE) != "pgsql") {
+				if ($pref_name == Prefs::DEFAULT_SEARCH_LANGUAGE && Config::get(Config::DB_TYPE) != "pgsql") {
 					continue;
 				}
 
@@ -596,17 +609,17 @@ class Pref_Prefs extends Handler_Protected {
 					$value = $item['value'];
 					$type_hint = $item['type_hint'];
 
-					if ($pref_name == "USER_LANGUAGE") {
+					if ($pref_name == Prefs::USER_LANGUAGE) {
 						print \Controls\select_hash($pref_name, $value, get_translations(),
 							["style" => 'width : 220px; margin : 0px']);
 
-					} else if ($pref_name == "USER_TIMEZONE") {
+					} else if ($pref_name == Prefs::USER_TIMEZONE) {
 
 						$timezones = explode("\n", file_get_contents("lib/timezones.txt"));
 
 						print \Controls\select_tag($pref_name, $value, $timezones, ["dojoType" => "dijit.form.FilteringSelect"]);
 
-					} else if ($pref_name == "BLACKLISTED_TAGS") { # TODO: other possible <textarea> prefs go here
+					} else if ($pref_name == Prefs::BLACKLISTED_TAGS) { # TODO: other possible <textarea> prefs go here
 
 						print "<div>";
 
@@ -618,7 +631,7 @@ class Pref_Prefs extends Handler_Protected {
 
 						print "</div>";
 
-					} else if ($pref_name == "USER_CSS_THEME") {
+					} else if ($pref_name == Prefs::USER_CSS_THEME) {
 
 						$theme_files = array_map("basename",
 							array_merge(glob("themes/*.php"),
@@ -642,13 +655,13 @@ class Pref_Prefs extends Handler_Protected {
 
 						<?php
 
-					} else if ($pref_name == "DEFAULT_UPDATE_INTERVAL") {
+					} else if ($pref_name == Prefs::DEFAULT_UPDATE_INTERVAL) {
 
 						global $update_intervals_nodefault;
 
 						print \Controls\select_hash($pref_name, $value, $update_intervals_nodefault);
 
-					} else if ($pref_name == "DEFAULT_SEARCH_LANGUAGE") {
+					} else if ($pref_name == Prefs::DEFAULT_SEARCH_LANGUAGE) {
 
 						print \Controls\select_tag($pref_name, $value, Pref_Feeds::get_ts_languages());
 
@@ -656,7 +669,7 @@ class Pref_Prefs extends Handler_Protected {
 
 						array_push($listed_boolean_prefs, $pref_name);
 
-						if ($pref_name == "PURGE_UNREAD_ARTICLES" && Config::get(Config::FORCE_ARTICLE_PURGE) != 0) {
+						if ($pref_name == Prefs::PURGE_UNREAD_ARTICLES && Config::get(Config::FORCE_ARTICLE_PURGE) != 0) {
 							$is_disabled = true;
 							$is_checked = true;
 						} else {
@@ -672,10 +685,10 @@ class Pref_Prefs extends Handler_Protected {
 								['onclick' => 'Helpers.Digest.preview()', 'style' => 'margin-left : 10px']);
 						}
 
-					} else if (in_array($pref_name, ['FRESH_ARTICLE_MAX_AGE',
-							'PURGE_OLD_DAYS', 'LONG_DATE_FORMAT', 'SHORT_DATE_FORMAT'])) {
+					} else if (in_array($pref_name, [Prefs::FRESH_ARTICLE_MAX_AGE,
+							Prefs::PURGE_OLD_DAYS, Prefs::LONG_DATE_FORMAT, Prefs::SHORT_DATE_FORMAT])) {
 
-						if ($pref_name == "PURGE_OLD_DAYS" && Config::get(Config::FORCE_ARTICLE_PURGE) != 0) {
+						if ($pref_name == Prefs::PURGE_OLD_DAYS && Config::get(Config::FORCE_ARTICLE_PURGE) != 0) {
 							$attributes = ["disabled" => true, "required" => true];
 							$value = Config::get(Config::FORCE_ARTICLE_PURGE);
 						} else {
@@ -687,7 +700,7 @@ class Pref_Prefs extends Handler_Protected {
 						else
 							print \Controls\input_tag($pref_name, $value, "text", $attributes);
 
-					} else if ($pref_name == "SSL_CERT_SERIAL") {
+					} else if ($pref_name == Prefs::SSL_CERT_SERIAL) {
 
 						print \Controls\input_tag($pref_name, $value, "text", ["readonly" => true], "SSL_CERT_SERIAL");
 
@@ -727,7 +740,7 @@ class Pref_Prefs extends Handler_Protected {
 		print \Controls\hidden_tag("boolean_prefs", htmlspecialchars(join(",", $listed_boolean_prefs)));
 	}
 
-	private function index_prefs() {
+	private function index_prefs(): void {
 		?>
 		<form dojoType='dijit.form.Form' id='changeSettingsForm'>
 			<?= \Controls\hidden_tag("op", "pref-prefs") ?>
@@ -783,7 +796,7 @@ class Pref_Prefs extends Handler_Protected {
 		<?php
 	}
 
-	function getPluginsList() {
+	function getPluginsList(): void {
 		$system_enabled = array_map("trim", explode(",", (string)Config::get(Config::PLUGINS)));
 		$user_enabled = array_map("trim", explode(",", get_pref(Prefs::_ENABLED_PLUGINS)));
 
@@ -816,7 +829,7 @@ class Pref_Prefs extends Handler_Protected {
 		print json_encode(['plugins' => $rv, 'is_admin' => $_SESSION['access_level'] >= UserHelper::ACCESS_LEVEL_ADMIN]);
 	}
 
-	function index_plugins() {
+	function index_plugins(): void {
 		?>
 		<form dojoType="dijit.form.Form" id="changePluginsForm">
 
@@ -912,7 +925,7 @@ class Pref_Prefs extends Handler_Protected {
 		<?php
 	}
 
-	function index() {
+	function index(): void {
 		?>
 			<div dojoType='dijit.layout.AccordionContainer' region='center'>
 				<div dojoType='dijit.layout.AccordionPane' title="<i class='material-icons'>person</i> <?= __('Personal data / Authentication')?>">
@@ -937,7 +950,7 @@ class Pref_Prefs extends Handler_Protected {
 		<?php
 	}
 
-	function _get_otp_qrcode_img() {
+	function _get_otp_qrcode_img(): ?string {
 		$secret = UserHelper::get_otp_secret($_SESSION["uid"]);
 		$login = UserHelper::get_login_by_id($_SESSION["uid"]);
 
@@ -949,15 +962,16 @@ class Pref_Prefs extends Handler_Protected {
 			return $qrcode->render($otpurl);
 		}
 
-		return false;
+		return null;
 	}
 
-	function otpenable() {
+	function otpenable(): void {
 		$password = clean($_REQUEST["password"]);
 		$otp_check = clean($_REQUEST["otp"]);
 
 		$authenticator = PluginHost::getInstance()->get_plugin($_SESSION["auth_module"]);
 
+		/** @var Auth_Internal|false $authenticator -- this is only here to make check_password() visible to static analyzer */
 		if ($authenticator->check_password($_SESSION["uid"], $password)) {
 			if (UserHelper::enable_otp($_SESSION["uid"], $otp_check)) {
 				print "OK";
@@ -969,9 +983,10 @@ class Pref_Prefs extends Handler_Protected {
 		}
 	}
 
-	function otpdisable() {
+	function otpdisable(): void {
 		$password = clean($_REQUEST["password"]);
 
+		/** @var Auth_Internal|false $authenticator -- this is only here to make check_password() visible to static analyzer */
 		$authenticator = PluginHost::getInstance()->get_plugin($_SESSION["auth_module"]);
 
 		if ($authenticator->check_password($_SESSION["uid"], $password)) {
@@ -1008,38 +1023,42 @@ class Pref_Prefs extends Handler_Protected {
 
 	}
 
-	function setplugins() {
+	function setplugins(): void {
 		$plugins = array_filter($_REQUEST["plugins"], 'clean') ?? [];
 
 		set_pref(Prefs::_ENABLED_PLUGINS, implode(",", $plugins));
 	}
 
-	function _get_plugin_version(Plugin $plugin) {
+	function _get_plugin_version(Plugin $plugin): string {
 		$about = $plugin->about();
 
 		if (!empty($about[0])) {
 			return T_sprintf("v%.2f, by %s", $about[0], $about[2]);
-		} else {
-			$ref = new ReflectionClass(get_class($plugin));
+		}
 
-			$plugin_dir = dirname($ref->getFileName());
+		$ref = new ReflectionClass(get_class($plugin));
 
-			if (basename($plugin_dir) == "plugins") {
-				return "";
-			}
+		$plugin_dir = dirname($ref->getFileName());
 
-			if (is_dir("$plugin_dir/.git")) {
-				$ver = Config::get_version_from_git($plugin_dir);
+		if (basename($plugin_dir) == "plugins") {
+			return "";
+		}
 
-				return $ver["status"] == 0 ? T_sprintf("v%s, by %s", $ver["version"], $about[2]) : $ver["version"];
-			}
+		if (is_dir("$plugin_dir/.git")) {
+			$ver = Config::get_version_from_git($plugin_dir);
+
+			return $ver["status"] == 0 ? T_sprintf("v%s, by %s", $ver["version"], $about[2]) : $ver["version"];
 		}
+
+		return "";
 	}
 
-	static function _get_updated_plugins() {
+	/**
+	 * @return array<int, array{'plugin': string, 'rv': array{'stdout': false|string, 'stderr': false|string, 'git_status': int, 'need_update': bool}|null}>
+	 */
+	static function _get_updated_plugins(): array {
 		$root_dir = dirname(dirname(__DIR__)); # we're in classes/pref/
 		$plugin_dirs = array_filter(glob("$root_dir/plugins.local/*"), "is_dir");
-
 		$rv = [];
 
 		foreach ($plugin_dirs as $dir) {
@@ -1057,7 +1076,10 @@ class Pref_Prefs extends Handler_Protected {
 		return $rv;
 	}
 
-	private static function _plugin_needs_update($root_dir, $plugin_name) {
+	/**
+	 * @return array{'stdout': false|string, 'stderr': false|string, 'git_status': int, 'need_update': bool}|null
+	 */
+	private static function _plugin_needs_update(string $root_dir, string $plugin_name): ?array {
 		$plugin_dir = "$root_dir/plugins.local/" . basename($plugin_name);
 		$rv = null;
 
@@ -1086,7 +1108,10 @@ class Pref_Prefs extends Handler_Protected {
 	}
 
 
-	private function _update_plugin($root_dir, $plugin_name) {
+	/**
+	 * @return array{'stdout': false|string, 'stderr': false|string, 'git_status': int}
+	 */
+	private function _update_plugin(string $root_dir, string $plugin_name): array {
 		$plugin_dir = "$root_dir/plugins.local/" . basename($plugin_name);
 		$rv = [];
 
@@ -1112,7 +1137,7 @@ class Pref_Prefs extends Handler_Protected {
 	}
 
 	// https://gist.github.com/mindplay-dk/a4aad91f5a4f1283a5e2#gistcomment-2036828
-	private function _recursive_rmdir(string $dir, bool $keep_root = false) {
+	private function _recursive_rmdir(string $dir, bool $keep_root = false): bool {
 		// Handle bad arguments.
 		if (empty($dir) || !file_exists($dir)) {
 			 return true; // No such file/dir$dir exists.
@@ -1137,7 +1162,7 @@ class Pref_Prefs extends Handler_Protected {
 	}
 
 	// https://stackoverflow.com/questions/7153000/get-class-name-from-file
-	private function _get_class_name_from_file($file) {
+	private function _get_class_name_from_file(string $file): string {
 		$tokens = token_get_all(file_get_contents($file));
 
 		for ($i = 0; $i < count($tokens); $i++) {
@@ -1149,9 +1174,11 @@ class Pref_Prefs extends Handler_Protected {
 				}
 			}
 		}
+
+		return "";
 	}
 
-	function uninstallPlugin() {
+	function uninstallPlugin():  void {
 		if ($_SESSION["access_level"] >= UserHelper::ACCESS_LEVEL_ADMIN) {
 			$plugin_name = basename(clean($_REQUEST['plugin']));
 			$status = 0;
@@ -1166,7 +1193,7 @@ class Pref_Prefs extends Handler_Protected {
 		}
 	}
 
-	function installPlugin() {
+	function installPlugin(): void {
 		if ($_SESSION["access_level"] >= UserHelper::ACCESS_LEVEL_ADMIN && Config::get(Config::ENABLE_PLUGIN_INSTALLER)) {
 			$plugin_name = basename(clean($_REQUEST['plugin']));
 			$all_plugins = $this->_get_available_plugins();
@@ -1251,47 +1278,59 @@ class Pref_Prefs extends Handler_Protected {
 		}
 	}
 
-	private function _get_available_plugins() {
+	/**
+	 * @return array<int, array{'name': string, 'description': string, 'topics': array<int, string>, 'html_url': string, 'clone_url': string, 'last_update': string}>
+	 */
+	private function _get_available_plugins(): array {
 		if ($_SESSION["access_level"] >= UserHelper::ACCESS_LEVEL_ADMIN && Config::get(Config::ENABLE_PLUGIN_INSTALLER)) {
-			return json_decode(UrlHelper::fetch(['url' => 'https://tt-rss.org/plugins.json']), true);
+			$content = json_decode(UrlHelper::fetch(['url' => 'https://tt-rss.org/plugins.json']), true);
+
+			if ($content) {
+				return $content;
+			}
 		}
+
+		return [];
 	}
-	function getAvailablePlugins() {
+
+	function getAvailablePlugins(): void {
 		if ($_SESSION["access_level"] >= UserHelper::ACCESS_LEVEL_ADMIN) {
 			print json_encode($this->_get_available_plugins());
+		} else {
+			print "[]";
 		}
 	}
 
-	function checkForPluginUpdates() {
+	function checkForPluginUpdates(): void {
 		if ($_SESSION["access_level"] >= UserHelper::ACCESS_LEVEL_ADMIN && Config::get(Config::CHECK_FOR_UPDATES) && Config::get(Config::CHECK_FOR_PLUGIN_UPDATES)) {
 			$plugin_name = $_REQUEST["name"] ?? "";
-
 			$root_dir = dirname(dirname(__DIR__)); # we're in classes/pref/
 
-			if (!empty($plugin_name)) {
-				$rv = [["plugin" => $plugin_name, "rv" => self::_plugin_needs_update($root_dir, $plugin_name)]];
-			} else {
-				$rv = self::_get_updated_plugins();
-			}
+			$rv = empty($plugin_name) ? self::_get_updated_plugins() : [
+				["plugin" => $plugin_name, "rv" => self::_plugin_needs_update($root_dir, $plugin_name)],
+			];
 
 			print json_encode($rv);
 		}
 	}
 
-	function updateLocalPlugins() {
+	function updateLocalPlugins(): void {
 		if ($_SESSION["access_level"] >= UserHelper::ACCESS_LEVEL_ADMIN) {
 			$plugins = explode(",", $_REQUEST["plugins"] ?? "");
 
+			if ($plugins !== false) {
+				$plugins = array_filter($plugins, 'strlen');
+			}
+
 			# we're in classes/pref/
 			$root_dir = dirname(dirname(__DIR__));
 
 			$rv = [];
 
-			if (count($plugins) > 0) {
+			if ($plugins) {
 				foreach ($plugins as $plugin_name) {
 					array_push($rv, ["plugin" => $plugin_name, "rv" => $this->_update_plugin($root_dir, $plugin_name)]);
 				}
-			// @phpstan-ignore-next-line
 			} else {
 				$plugin_dirs = array_filter(glob("$root_dir/plugins.local/*"), "is_dir");
 
@@ -1301,7 +1340,7 @@ class Pref_Prefs extends Handler_Protected {
 
 						$test = self::_plugin_needs_update($root_dir, $plugin_name);
 
-						if (!empty($test["o"]))
+						if (!empty($test["stdout"]))
 							array_push($rv, ["plugin" => $plugin_name, "rv" => $this->_update_plugin($root_dir, $plugin_name)]);
 					}
 				}
@@ -1311,20 +1350,20 @@ class Pref_Prefs extends Handler_Protected {
 		}
 	}
 
-	function clearplugindata() {
+	function clearplugindata(): void {
 		$name = clean($_REQUEST["name"]);
 
 		PluginHost::getInstance()->clear_data(PluginHost::getInstance()->get_plugin($name));
 	}
 
-	function customizeCSS() {
+	function customizeCSS(): void {
 		$value = get_pref(Prefs::USER_STYLESHEET);
 		$value = str_replace("<br/>", "\n", $value);
 
 		print json_encode(["value" => $value]);
 	}
 
-	function activateprofile() {
+	function activateprofile(): void {
 		$id = (int) ($_REQUEST['id'] ?? 0);
 
 		$profile = ORM::for_table('ttrss_settings_profiles')
@@ -1338,7 +1377,7 @@ class Pref_Prefs extends Handler_Protected {
 		}
 	}
 
-	function cloneprofile() {
+	function cloneprofile(): void {
 		$old_profile = $_REQUEST["old_profile"] ?? 0;
 		$new_title = clean($_REQUEST["new_title"]);
 
@@ -1367,7 +1406,7 @@ class Pref_Prefs extends Handler_Protected {
 		}
 	}
 
-	function remprofiles() {
+	function remprofiles(): void {
 		$ids = $_REQUEST["ids"] ?? [];
 
 		ORM::for_table('ttrss_settings_profiles')
@@ -1377,7 +1416,7 @@ class Pref_Prefs extends Handler_Protected {
 			->delete_many();
 	}
 
-	function addprofile() {
+	function addprofile(): void {
 		$title = clean($_REQUEST["title"]);
 
 		if ($title) {
@@ -1396,7 +1435,7 @@ class Pref_Prefs extends Handler_Protected {
 		}
 	}
 
-	function saveprofile() {
+	function saveprofile(): void {
 		$id = (int)$_REQUEST["id"];
 		$title = clean($_REQUEST["value"]);
 
@@ -1413,7 +1452,7 @@ class Pref_Prefs extends Handler_Protected {
 	}
 
 	// TODO: this maybe needs to be unified with Public::getProfiles()
-	function getProfiles() {
+	function getProfiles(): void {
 		$rv = [];
 
 		$profiles = ORM::for_table('ttrss_settings_profiles')
@@ -1442,21 +1481,21 @@ class Pref_Prefs extends Handler_Protected {
 		print json_encode($rv);
 	}
 
-	private function _get_short_desc($pref_name) {
+	private function _get_short_desc(string $pref_name): string {
 		if (isset($this->pref_help[$pref_name][0])) {
 			return $this->pref_help[$pref_name][0];
 		}
 		return "";
 	}
 
-	private function _get_help_text($pref_name) {
+	private function _get_help_text(string $pref_name): string {
 		if (isset($this->pref_help[$pref_name][1])) {
 			return $this->pref_help[$pref_name][1];
 		}
 		return "";
 	}
 
-	private function appPasswordList() {
+	private function appPasswordList(): void {
 		?>
 		<div dojoType='fox.Toolbar'>
 			<div dojoType='fox.form.DropDownButton'>
@@ -1506,7 +1545,7 @@ class Pref_Prefs extends Handler_Protected {
 		<?php
 	}
 
-	function deleteAppPasswords() {
+	function deleteAppPasswords(): void {
 		$passwords = ORM::for_table('ttrss_app_passwords')
 			->where('owner_uid', $_SESSION['uid'])
 			->where_in('id', $_REQUEST['ids'] ?? [])
@@ -1515,7 +1554,7 @@ class Pref_Prefs extends Handler_Protected {
 		$this->appPasswordList();
 	}
 
-	function generateAppPassword() {
+	function generateAppPassword(): void {
 		$title = clean($_REQUEST['title']);
 		$new_password = make_password(16);
 		$new_salt = UserHelper::get_salt();
@@ -1536,11 +1575,11 @@ class Pref_Prefs extends Handler_Protected {
 		$this->appPasswordList();
 	}
 
-	function previewDigest() {
+	function previewDigest(): void {
 		print json_encode(Digest::prepare_headlines_digest($_SESSION["uid"], 1, 16));
 	}
 
-	static function _get_ssl_certificate_id() {
+	static function _get_ssl_certificate_id(): string {
 		if ($_SERVER["REDIRECT_SSL_CLIENT_M_SERIAL"] ?? false) {
 			return sha1($_SERVER["REDIRECT_SSL_CLIENT_M_SERIAL"] .
 				$_SERVER["REDIRECT_SSL_CLIENT_V_START"] .
@@ -1556,7 +1595,7 @@ class Pref_Prefs extends Handler_Protected {
 		return "";
 	}
 
-	private function format_otp_secret($secret) {
+	private function format_otp_secret(string $secret): string {
 		return implode(" ", str_split($secret, 4));
 	}
 }
diff --git a/classes/pref/system.php b/classes/pref/system.php
index 8bebcc7ce..10f196b55 100644
--- a/classes/pref/system.php
+++ b/classes/pref/system.php
@@ -2,19 +2,19 @@
 
 class Pref_System extends Handler_Administrative {
 
-	private $log_page_limit = 15;
+	private const LOG_PAGE_LIMIT = 15;
 
-	function csrf_ignore($method) {
+	function csrf_ignore(string $method): bool {
 		$csrf_ignored = array("index");
 
 		return array_search($method, $csrf_ignored) !== false;
 	}
 
-	function clearLog() {
+	function clearLog(): void {
 		$this->pdo->query("DELETE FROM ttrss_error_log");
 	}
 
-	function sendTestEmail() {
+	function sendTestEmail(): void {
 		$mail_address = clean($_REQUEST["mail_address"]);
 
 		$mailer = new Mailer();
@@ -28,7 +28,7 @@ class Pref_System extends Handler_Administrative {
 		print json_encode(['rc' => $rc, 'error' => $mailer->error()]);
 	}
 
-	function getphpinfo() {
+	function getphpinfo(): void {
 		ob_start();
 		phpinfo();
 		$info = ob_get_contents();
@@ -37,7 +37,7 @@ class Pref_System extends Handler_Administrative {
 		print preg_replace( '%^.*<body>(.*)</body>.*$%ms','$1', (string)$info);
 	}
 
-	private function _log_viewer(int $page, int $severity) {
+	private function _log_viewer(int $page, int $severity): void {
 		$errno_values = [];
 
 		switch ($severity) {
@@ -56,8 +56,7 @@ class Pref_System extends Handler_Administrative {
 			$errno_filter_qpart = "true";
 		}
 
-		$limit = $this->log_page_limit;
-		$offset = $limit * $page;
+		$offset = self::LOG_PAGE_LIMIT * $page;
 
 		$sth = $this->pdo->prepare("SELECT
 				COUNT(id) AS total_pages
@@ -69,7 +68,7 @@ class Pref_System extends Handler_Administrative {
 		$sth->execute($errno_values);
 
 		if ($res = $sth->fetch()) {
-			$total_pages = (int)($res["total_pages"] / $limit);
+			$total_pages = (int)($res["total_pages"] / self::LOG_PAGE_LIMIT);
 		} else {
 			$total_pages = 0;
 		}
@@ -134,7 +133,7 @@ class Pref_System extends Handler_Administrative {
 							$errno_filter_qpart
 						ORDER BY
 							ttrss_error_log.id DESC
-						LIMIT $limit OFFSET $offset");
+						LIMIT ". self::LOG_PAGE_LIMIT ." OFFSET $offset");
 
 					$sth->execute($errno_values);
 
@@ -159,7 +158,7 @@ class Pref_System extends Handler_Administrative {
 		<?php
 	}
 
-	function index() {
+	function index(): void {
 
 		$severity = (int) ($_REQUEST["severity"] ?? E_USER_WARNING);
 		$page = (int) ($_REQUEST["page"] ?? 0);
diff --git a/classes/pref/users.php b/classes/pref/users.php
index aeba296b2..c48619614 100644
--- a/classes/pref/users.php
+++ b/classes/pref/users.php
@@ -1,10 +1,10 @@
 <?php
 class Pref_Users extends Handler_Administrative {
-		function csrf_ignore($method) {
+		function csrf_ignore(string $method): bool {
 			return $method == "index";
 		}
 
-		function edit() {
+		function edit(): void {
 			$user = ORM::for_table('ttrss_users')
 				->select_expr("id,login,access_level,email,full_name,otp_enabled")
 				->find_one((int)$_REQUEST["id"])
@@ -20,7 +20,7 @@ class Pref_Users extends Handler_Administrative {
 			}
 		}
 
-		function userdetails() {
+		function userdetails(): void {
 			$id = (int) clean($_REQUEST["id"]);
 
 			$sth = $this->pdo->prepare("SELECT login,
@@ -103,7 +103,7 @@ class Pref_Users extends Handler_Administrative {
 
 		}
 
-		function editSave() {
+		function editSave(): void {
 			$id = (int)$_REQUEST['id'];
 			$password = clean($_REQUEST["password"]);
 			$user = ORM::for_table('ttrss_users')->find_one($id);
@@ -132,7 +132,7 @@ class Pref_Users extends Handler_Administrative {
 			}
 		}
 
-		function remove() {
+		function remove(): void {
 			$ids = explode(",", clean($_REQUEST["ids"]));
 
 			foreach ($ids as $id) {
@@ -149,7 +149,7 @@ class Pref_Users extends Handler_Administrative {
 			}
 		}
 
-		function add() {
+		function add(): void {
 			$login = clean($_REQUEST["login"]);
 
 			if (!$login) return; // no blank usernames
@@ -178,11 +178,11 @@ class Pref_Users extends Handler_Administrative {
 			}
 		}
 
-		function resetPass() {
+		function resetPass(): void {
 			UserHelper::reset_password(clean($_REQUEST["id"]));
 		}
 
-		function index() {
+		function index(): void {
 
 			global $access_level_names;
 
diff --git a/classes/prefs.php b/classes/prefs.php
index 85e7c34db..7e6033f4d 100644
--- a/classes/prefs.php
+++ b/classes/prefs.php
@@ -141,7 +141,10 @@ class Prefs {
 		Prefs::_PREFS_MIGRATED
 	];
 
+	/** @var Prefs|null */
 	private static $instance;
+
+	/** @var array<string, bool|int|string> */
 	private $cache = [];
 
 	/** @var PDO */
@@ -154,10 +157,13 @@ class Prefs {
 		return self::$instance;
 	}
 
-	static function is_valid(string $pref_name) {
+	static function is_valid(string $pref_name): bool {
 		return isset(self::_DEFAULTS[$pref_name]);
 	}
 
+	/**
+	 * @return bool|int|null|string
+	 */
 	static function get_default(string $pref_name) {
 		if (self::is_valid($pref_name))
 			return self::_DEFAULTS[$pref_name][0];
@@ -181,10 +187,16 @@ class Prefs {
 		//
 	}
 
+	/**
+	 * @return array<int, array<string, bool|int|null|string>>
+	 */
 	static function get_all(int $owner_uid, int $profile_id = null) {
 		return self::get_instance()->_get_all($owner_uid, $profile_id);
 	}
 
+	/**
+	 * @return array<int, array<string, bool|int|null|string>>
+	 */
 	private function _get_all(int $owner_uid, int $profile_id = null) {
 		$rv = [];
 
@@ -205,7 +217,7 @@ class Prefs {
 		return $rv;
 	}
 
-	private function cache_all(int $owner_uid, $profile_id = null) {
+	private function cache_all(int $owner_uid, ?int $profile_id): void {
 		if (!$profile_id) $profile_id = null;
 
 		// fill cache with defaults
@@ -232,11 +244,17 @@ class Prefs {
 		}
 	}
 
-	static function get(string $pref_name, int $owner_uid, int $profile_id = null) {
+	/**
+	 * @return bool|int|null|string
+	 */
+	static function get(string $pref_name, int $owner_uid, ?int $profile_id) {
 		return self::get_instance()->_get($pref_name, $owner_uid, $profile_id);
 	}
 
-	private function _get(string $pref_name, int $owner_uid, int $profile_id = null) {
+	/**
+	 * @return bool|int|null|string
+	 */
+	private function _get(string $pref_name, int $owner_uid, ?int $profile_id) {
 		if (isset(self::_DEFAULTS[$pref_name])) {
 			if (!$profile_id || in_array($pref_name, self::_PROFILE_BLACKLIST)) $profile_id = null;
 
@@ -274,12 +292,15 @@ class Prefs {
 		return null;
 	}
 
-	private function _is_cached(string $pref_name, int $owner_uid, int $profile_id = null) {
+	private function _is_cached(string $pref_name, int $owner_uid, ?int $profile_id): bool {
 		$cache_key = sprintf("%d/%d/%s", $owner_uid, $profile_id, $pref_name);
 		return isset($this->cache[$cache_key]);
 	}
 
-	private function _get_cache(string $pref_name, int $owner_uid, int $profile_id = null) {
+	/**
+	 * @return bool|int|null|string
+	 */
+	private function _get_cache(string $pref_name, int $owner_uid, ?int $profile_id) {
 		$cache_key = sprintf("%d/%d/%s", $owner_uid, $profile_id, $pref_name);
 
 		if (isset($this->cache[$cache_key]))
@@ -288,17 +309,23 @@ class Prefs {
 		return null;
 	}
 
-	private function _set_cache(string $pref_name, $value, int $owner_uid, int $profile_id = null) {
+	/**
+	 * @param bool|int|string $value
+	 */
+	private function _set_cache(string $pref_name, $value, int $owner_uid, ?int $profile_id): void {
 		$cache_key = sprintf("%d/%d/%s", $owner_uid, $profile_id, $pref_name);
 
 		$this->cache[$cache_key] = $value;
 	}
 
-	static function set(string $pref_name, $value, int $owner_uid, int $profile_id = null, bool $strip_tags = true) {
+	/**
+	 * @param bool|int|string $value
+	 */
+	static function set(string $pref_name, $value, int $owner_uid, ?int $profile_id, bool $strip_tags = true): bool {
 		return self::get_instance()->_set($pref_name, $value, $owner_uid, $profile_id);
 	}
 
-	private function _delete(string $pref_name, int $owner_uid, int $profile_id = null) {
+	private function _delete(string $pref_name, int $owner_uid, ?int $profile_id): bool {
 		$sth = $this->pdo->prepare("DELETE FROM ttrss_user_prefs2
 			WHERE pref_name = :name AND owner_uid = :uid AND
 				(profile = :profile OR (:profile IS NULL AND profile IS NULL))");
@@ -306,7 +333,10 @@ class Prefs {
 		return $sth->execute(["uid" => $owner_uid, "profile" => $profile_id, "name" => $pref_name ]);
 	}
 
-	private function _set(string $pref_name, $value, int $owner_uid, int $profile_id = null, bool $strip_tags = true) {
+	/**
+	 * @param bool|int|string $value
+	 */
+	private function _set(string $pref_name, $value, int $owner_uid, ?int $profile_id, bool $strip_tags = true): bool {
 		if (!$profile_id) $profile_id = null;
 
 		if ($profile_id && in_array($pref_name, self::_PROFILE_BLACKLIST))
@@ -359,7 +389,7 @@ class Prefs {
 		return false;
 	}
 
-	function migrate(int $owner_uid, int $profile_id = null) {
+	function migrate(int $owner_uid, ?int $profile_id): void {
 		if (get_schema_version() < 141)
 			return;
 
@@ -401,7 +431,7 @@ class Prefs {
 		}
 	}
 
-	static function reset(int $owner_uid, int $profile_id = null) {
+	static function reset(int $owner_uid, ?int $profile_id): void {
 		if (!$profile_id) $profile_id = null;
 
 		$sth = Db::pdo()->prepare("DELETE FROM ttrss_user_prefs2
diff --git a/classes/rpc.php b/classes/rpc.php
index 0432ed2d3..75d008b8b 100755
--- a/classes/rpc.php
+++ b/classes/rpc.php
@@ -1,13 +1,16 @@
 <?php
 class RPC extends Handler_Protected {
 
-	/*function csrf_ignore($method) {
+	/*function csrf_ignore(string $method): bool {
 		$csrf_ignored = array("completelabels");
 
 		return array_search($method, $csrf_ignored) !== false;
 	}*/
 
-	private function _translations_as_array() {
+	/**
+	 * @return array<string, string>
+	 */
+	private function _translations_as_array(): array {
 
 		global $text_domains;
 
@@ -37,7 +40,7 @@ class RPC extends Handler_Protected {
 	}
 
 
-	function togglepref() {
+	function togglepref(): void {
 		$key = clean($_REQUEST["key"]);
 		set_pref($key, !get_pref($key));
 		$value = get_pref($key);
@@ -45,7 +48,7 @@ class RPC extends Handler_Protected {
 		print json_encode(array("param" =>$key, "value" => $value));
 	}
 
-	function setpref() {
+	function setpref(): void {
 		// set_pref escapes input, so no need to double escape it here
 		$key = clean($_REQUEST['key']);
 		$value = $_REQUEST['value'];
@@ -55,7 +58,7 @@ class RPC extends Handler_Protected {
 		print json_encode(array("param" =>$key, "value" => $value));
 	}
 
-	function mark() {
+	function mark(): void {
 		$mark = clean($_REQUEST["mark"]);
 		$id = clean($_REQUEST["id"]);
 
@@ -68,7 +71,7 @@ class RPC extends Handler_Protected {
 		print json_encode(array("message" => "UPDATE_COUNTERS"));
 	}
 
-	function delete() {
+	function delete(): void {
 		$ids = explode(",", clean($_REQUEST["ids"]));
 		$ids_qmarks = arr_qmarks($ids);
 
@@ -81,7 +84,7 @@ class RPC extends Handler_Protected {
 		print json_encode(array("message" => "UPDATE_COUNTERS"));
 	}
 
-	function publ() {
+	function publ(): void {
 		$pub = clean($_REQUEST["pub"]);
 		$id = clean($_REQUEST["id"]);
 
@@ -94,7 +97,7 @@ class RPC extends Handler_Protected {
 		print json_encode(array("message" => "UPDATE_COUNTERS"));
 	}
 
-	function getRuntimeInfo() {
+	function getRuntimeInfo(): void {
 		$reply = [
 			'runtime-info' => $this->_make_runtime_info()
 		];
@@ -102,7 +105,7 @@ class RPC extends Handler_Protected {
 		print json_encode($reply);
 	}
 
-	function getAllCounters() {
+	function getAllCounters(): void {
 		@$seq = (int) $_REQUEST['seq'];
 
 		$feed_id_count = (int)$_REQUEST["feed_id_count"];
@@ -133,7 +136,7 @@ class RPC extends Handler_Protected {
 	}
 
 	/* GET["cmode"] = 0 - mark as read, 1 - as unread, 2 - toggle */
-	function catchupSelected() {
+	function catchupSelected(): void {
 		$ids = array_map("intval", clean($_REQUEST["ids"] ?? []));
 		$cmode = (int)clean($_REQUEST["cmode"]);
 
@@ -145,7 +148,7 @@ class RPC extends Handler_Protected {
 			"feeds" => Article::_feeds_of($ids)]);
 	}
 
-	function markSelected() {
+	function markSelected(): void {
 		$ids = array_map("intval", clean($_REQUEST["ids"] ?? []));
 		$cmode = (int)clean($_REQUEST["cmode"]);
 
@@ -157,7 +160,7 @@ class RPC extends Handler_Protected {
 			"feeds" => Article::_feeds_of($ids)]);
 	}
 
-	function publishSelected() {
+	function publishSelected(): void {
 		$ids = array_map("intval", clean($_REQUEST["ids"] ?? []));
 		$cmode = (int)clean($_REQUEST["cmode"]);
 
@@ -169,7 +172,7 @@ class RPC extends Handler_Protected {
 			"feeds" => Article::_feeds_of($ids)]);
 	}
 
-	function sanityCheck() {
+	function sanityCheck(): void {
 		$_SESSION["hasSandbox"] = clean($_REQUEST["hasSandbox"]) === "true";
 		$_SESSION["clientTzOffset"] = clean($_REQUEST["clientTzOffset"]);
 
@@ -220,14 +223,14 @@ class RPC extends Handler_Protected {
 		print "</ul>";
 	}*/
 
-	function catchupFeed() {
+	function catchupFeed(): void {
 		$feed_id = clean($_REQUEST['feed_id']);
 		$is_cat = clean($_REQUEST['is_cat']) == "true";
 		$mode = clean($_REQUEST['mode'] ?? '');
 		$search_query = clean($_REQUEST['search_query']);
 		$search_lang = clean($_REQUEST['search_lang']);
 
-		Feeds::_catchup($feed_id, $is_cat, false, $mode, [$search_query, $search_lang]);
+		Feeds::_catchup($feed_id, $is_cat, null, $mode, [$search_query, $search_lang]);
 
 		// return counters here synchronously so that frontend can figure out next unread feed properly
 		print json_encode(['counters' => Counters::get_all()]);
@@ -235,7 +238,7 @@ class RPC extends Handler_Protected {
 		//print json_encode(array("message" => "UPDATE_COUNTERS"));
 	}
 
-	function setWidescreen() {
+	function setWidescreen(): void {
 		$wide = (int) clean($_REQUEST["wide"]);
 
 		set_pref(Prefs::WIDESCREEN_MODE, $wide);
@@ -243,7 +246,7 @@ class RPC extends Handler_Protected {
 		print json_encode(["wide" => $wide]);
 	}
 
-	static function updaterandomfeed_real() {
+	static function updaterandomfeed_real(): void {
 
 		$default_interval = (int) Prefs::get_default(Prefs::DEFAULT_UPDATE_INTERVAL);
 
@@ -336,19 +339,22 @@ class RPC extends Handler_Protected {
 
 	}
 
-	function updaterandomfeed() {
+	function updaterandomfeed(): void {
 		self::updaterandomfeed_real();
 	}
 
-	private function markArticlesById($ids, $cmode) {
+	/**
+	 * @param array<int, int> $ids
+	 */
+	private function markArticlesById(array $ids, int $cmode): void {
 
 		$ids_qmarks = arr_qmarks($ids);
 
-		if ($cmode == 0) {
+		if ($cmode == Article::CATCHUP_MODE_MARK_AS_READ) {
 			$sth = $this->pdo->prepare("UPDATE ttrss_user_entries SET
 				marked = false, last_marked = NOW()
 					WHERE ref_id IN ($ids_qmarks) AND owner_uid = ?");
-		} else if ($cmode == 1) {
+		} else if ($cmode == Article::CATCHUP_MODE_MARK_AS_UNREAD) {
 			$sth = $this->pdo->prepare("UPDATE ttrss_user_entries SET
 				marked = true, last_marked = NOW()
 					WHERE ref_id IN ($ids_qmarks) AND owner_uid = ?");
@@ -361,15 +367,18 @@ class RPC extends Handler_Protected {
 		$sth->execute(array_merge($ids, [$_SESSION['uid']]));
 	}
 
-	private function publishArticlesById($ids, $cmode) {
+	/**
+	 * @param array<int, int> $ids
+	 */
+	private function publishArticlesById(array $ids, int $cmode): void {
 
 		$ids_qmarks = arr_qmarks($ids);
 
-		if ($cmode == 0) {
+		if ($cmode == Article::CATCHUP_MODE_MARK_AS_READ) {
 			$sth = $this->pdo->prepare("UPDATE ttrss_user_entries SET
 				published = false, last_published = NOW()
 					WHERE ref_id IN ($ids_qmarks) AND owner_uid = ?");
-		} else if ($cmode == 1) {
+		} else if ($cmode == Article::CATCHUP_MODE_MARK_AS_UNREAD) {
 			$sth = $this->pdo->prepare("UPDATE ttrss_user_entries SET
 				published = true, last_published = NOW()
 					WHERE ref_id IN ($ids_qmarks) AND owner_uid = ?");
@@ -382,7 +391,7 @@ class RPC extends Handler_Protected {
 		$sth->execute(array_merge($ids, [$_SESSION['uid']]));
 	}
 
-	function log() {
+	function log(): void {
 		$msg = clean($_REQUEST['msg'] ?? "");
 		$file = basename(clean($_REQUEST['file'] ?? ""));
 		$line = (int) clean($_REQUEST['line'] ?? 0);
@@ -396,7 +405,7 @@ class RPC extends Handler_Protected {
 		}
 	}
 
-	function checkforupdates() {
+	function checkforupdates(): void {
 		$rv = ["changeset" => [], "plugins" => []];
 
 		$version = Config::get_version(false);
@@ -425,7 +434,10 @@ class RPC extends Handler_Protected {
 		print json_encode($rv);
 	}
 
-	private function _make_init_params() {
+	/**
+	 * @return array<string, mixed>
+	 */
+	private function _make_init_params(): array {
 		$params = array();
 
 		foreach ([Prefs::ON_CATCHUP_SHOW_NEXT_FEED, Prefs::HIDE_READ_FEEDS,
@@ -481,7 +493,7 @@ class RPC extends Handler_Protected {
 		return $params;
 	}
 
-	private function image_to_base64($filename) {
+	private function image_to_base64(string $filename): string {
 		if (file_exists($filename)) {
 			$ext = pathinfo($filename, PATHINFO_EXTENSION);
 
@@ -493,7 +505,10 @@ class RPC extends Handler_Protected {
 		}
 	}
 
-	static function _make_runtime_info() {
+	/**
+	 * @return array<string, mixed>
+	 */
+	static function _make_runtime_info(): array {
 		$data = array();
 
 		$pdo = Db::pdo();
@@ -562,7 +577,10 @@ class RPC extends Handler_Protected {
 		return $data;
 	}
 
-	static function get_hotkeys_info() {
+	/**
+	 * @return array<string, array<string, string>>
+	 */
+	static function get_hotkeys_info(): array {
 		$hotkeys = array(
 			__("Navigation") => array(
 				"next_feed" => __("Open next feed"),
@@ -642,8 +660,12 @@ class RPC extends Handler_Protected {
 		return $hotkeys;
 	}
 
-	// {3} - 3 panel mode only
-	// {C} - combined mode only
+	/**
+	 * {3} - 3 panel mode only
+	 * {C} - combined mode only
+	 *
+	 * @return array{0: array<int, string>, 1: array<string, string>} $prefixes, $hotkeys
+	 */
 	static function get_hotkeys_map() {
 		$hotkeys = array(
 			"k" => "next_feed",
@@ -728,7 +750,7 @@ class RPC extends Handler_Protected {
 		return array($prefixes, $hotkeys);
 	}
 
-	function hotkeyHelp() {
+	function hotkeyHelp(): void {
 		$info = self::get_hotkeys_info();
 		$imap = self::get_hotkeys_map();
 		$omap = array();
diff --git a/classes/rssutils.php b/classes/rssutils.php
index 927a6c251..b886a060c 100755
--- a/classes/rssutils.php
+++ b/classes/rssutils.php
@@ -1,6 +1,9 @@
 <?php
 class RSSUtils {
-	static function calculate_article_hash($article, $pluginhost) {
+	/**
+	 * @param array<string, mixed> $article
+	 */
+	static function calculate_article_hash(array $article, PluginHost $pluginhost): string {
 		$tmp = "";
 
 		$ignored_fields = [ "feed", "guid", "guid_hashed", "owner_uid", "force_catchup" ];
@@ -21,16 +24,16 @@ class RSSUtils {
 	}
 
 	// Strips utf8mb4 characters (i.e. emoji) for mysql
-	static function strip_utf8mb4(string $str) {
+	static function strip_utf8mb4(string $str): string {
 		return preg_replace('/[\x{10000}-\x{10FFFF}]/u', "\xEF\xBF\xBD", $str);
 	}
 
-	static function cleanup_feed_browser() {
+	static function cleanup_feed_browser(): void {
 		$pdo = Db::pdo();
 		$pdo->query("DELETE FROM ttrss_feedbrowser_cache");
 	}
 
-	static function cleanup_feed_icons() {
+	static function cleanup_feed_icons(): void {
 		$pdo = Db::pdo();
 		$sth = $pdo->prepare("SELECT id FROM ttrss_feeds WHERE id = ?");
 
@@ -52,7 +55,10 @@ class RSSUtils {
 		}
 	}
 
-	static function update_daemon_common(int $limit = 0, array $options = []) {
+	/**
+	 * @param array<string, false|string> $options
+	 */
+	static function update_daemon_common(int $limit = 0, array $options = []): int {
 		if (!$limit) $limit = Config::get(Config::DAEMON_FEED_LIMIT);
 
 		if (Config::get_schema_version() != Config::SCHEMA_VERSION) {
@@ -272,7 +278,7 @@ class RSSUtils {
 	}
 
 	/** this is used when subscribing */
-	static function update_basic_info(int $feed_id) {
+	static function update_basic_info(int $feed_id): void {
 		$feed = ORM::for_table('ttrss_feeds')
 			->select_many('id', 'owner_uid', 'feed_url', 'auth_pass', 'auth_login', 'title', 'site_url')
 			->find_one($feed_id);
@@ -681,7 +687,7 @@ class RSSUtils {
 				}
 
 				$entry_comments = mb_substr(strip_tags($item->get_comments_url()), 0, 245);
-				$num_comments = (int) $item->get_comments_count();
+				$num_comments = $item->get_comments_count();
 
 				$entry_author = strip_tags($item->get_author());
 				$entry_guid = mb_substr($entry_guid, 0, 245);
@@ -728,6 +734,7 @@ class RSSUtils {
 							},
 							$e, $feed);
 
+						// TODO: Just use FeedEnclosure (and modify it to cover whatever justified this)?
 						$e_item = array(
 							rewrite_relative_url($site_url, $e->link),
 							$e->type, $e->length, $e->title, $e->width, $e->height);
@@ -1265,8 +1272,14 @@ class RSSUtils {
 		return true;
 	}
 
-	/* TODO: move to DiskCache? */
-	static function cache_enclosures($enclosures, $site_url) {
+	/**
+	 * TODO: move to DiskCache?
+	 *
+	 * @param array<int, array<string>> $enclosures An array of "enclosure arrays" [string $link, string $type, string $length, string, $title, string $width, string $height]
+	 * @see RSSUtils::update_rss_feed()
+	 * @see FeedEnclosure
+	 */
+	static function cache_enclosures(array $enclosures, string $site_url): void {
 		$cache = new DiskCache("images");
 
 		if ($cache->is_writable()) {
@@ -1298,7 +1311,7 @@ class RSSUtils {
 	}
 
 	/* TODO: move to DiskCache? */
-	static function cache_media_url($cache, $url, $site_url) {
+	static function cache_media_url(DiskCache $cache, string $url, string $site_url): void {
 		$url = rewrite_relative_url($site_url, $url);
 		$local_filename = sha1($url);
 
@@ -1322,7 +1335,7 @@ class RSSUtils {
 	}
 
 	/* TODO: move to DiskCache? */
-	static function cache_media($html, $site_url) {
+	static function cache_media(string $html, string $site_url): void {
 		$cache = new DiskCache("images");
 
 		if ($html && $cache->is_writable()) {
@@ -1351,7 +1364,7 @@ class RSSUtils {
 		}
 	}
 
-	static function expire_error_log() {
+	static function expire_error_log(): void {
 		Debug::log("Removing old error log entries...");
 
 		$pdo = Db::pdo();
@@ -1365,14 +1378,16 @@ class RSSUtils {
 		}
 	}
 
-	// deprecated; table not used
-	static function expire_feed_archive() {
+	/**
+	 * @deprecated table not used
+	 */
+	static function expire_feed_archive(): void {
 		$pdo = Db::pdo();
 
 		$pdo->query("DELETE FROM ttrss_archived_feeds");
 	}
 
-	static function expire_lock_files() {
+	static function expire_lock_files(): void {
 		Debug::log("Removing old lock files...", Debug::LOG_VERBOSE);
 
 		$num_deleted = 0;
@@ -1413,7 +1428,15 @@ class RSSUtils {
 		return $params;
 	} */
 
-	static function get_article_filters($filters, $title, $content, $link, $author, $tags, &$matched_rules = false, &$matched_filters = false) {
+	/**
+	 * @param array<int, array<string, mixed>> $filters
+	 * @param array<int, string> $tags
+	 * @param array<int, array<string, mixed>>|null $matched_rules
+	 * @param array<int, array<string, mixed>>|null $matched_filters
+	 *
+	 * @return array<int, array<string, string>> An array of filter action arrays with keys "type" and "param"
+	 */
+	static function get_article_filters(array $filters, string $title, string $content, string $link, string $author, array $tags, array &$matched_rules = null, array &$matched_filters = null): array {
 		$matches = array();
 
 		foreach ($filters as $filter) {
@@ -1497,16 +1520,26 @@ class RSSUtils {
 		return $matches;
 	}
 
-	static function find_article_filter($filters, $filter_name) {
+	/**
+	 * @param array<int, array<string, string>> $filters An array of filter action arrays with keys "type" and "param"
+	 *
+	 * @return array<string, string>|null A filter action array with keys "type" and "param"
+	 */
+	static function find_article_filter(array $filters, string $filter_name): ?array {
 		foreach ($filters as $f) {
 			if ($f["type"] == $filter_name) {
 				return $f;
 			};
 		}
-		return false;
+		return null;
 	}
 
-	static function find_article_filters($filters, $filter_name) {
+	/**
+	 * @param array<int, array<string, string>> $filters An array of filter action arrays with keys "type" and "param"
+	 *
+	 * @return array<int, array<string, string>> An array of filter action arrays with keys "type" and "param"
+	 */
+	static function find_article_filters(array $filters, string $filter_name): array {
 		$results = array();
 
 		foreach ($filters as $f) {
@@ -1517,7 +1550,10 @@ class RSSUtils {
 		return $results;
 	}
 
-	static function calculate_article_score($filters) {
+	/**
+	 * @param array<int, array<string, string>> $filters An array of filter action arrays with keys "type" and "param"
+	 */
+	static function calculate_article_score(array $filters): int {
 		$score = 0;
 
 		foreach ($filters as $f) {
@@ -1528,7 +1564,12 @@ class RSSUtils {
 		return $score;
 	}
 
-	static function labels_contains_caption($labels, $caption) {
+	/**
+	 * @param array<int, array<int, int|string>> $labels An array of label arrays like [int $feed_id, string $caption, string $fg_color, string $bg_color]
+	 *
+	 * @see Article::_get_labels()
+	 */
+	static function labels_contains_caption(array $labels, string $caption): bool {
 		foreach ($labels as $label) {
 			if ($label[1] == $caption) {
 				return true;
@@ -1538,7 +1579,11 @@ class RSSUtils {
 		return false;
 	}
 
-	static function assign_article_to_label_filters($id, $filters, $owner_uid, $article_labels) {
+	/**
+	 * @param array<int, array<string, string>> $filters An array of filter action arrays with keys "type" and "param"
+	 * @param array<int, array<int, int|string>> $article_labels An array of label arrays like [int $feed_id, string $caption, string $fg_color, string $bg_color]
+	 */
+	static function assign_article_to_label_filters(int $id, array $filters, int $owner_uid, $article_labels): void {
 		foreach ($filters as $f) {
 			if ($f["type"] == "label") {
 				if (!self::labels_contains_caption($article_labels, $f["param"])) {
@@ -1548,20 +1593,20 @@ class RSSUtils {
 		}
 	}
 
-	static function make_guid_from_title($title) {
+	static function make_guid_from_title(string $title): ?string {
 		return preg_replace("/[ \"\',.:;]/", "-",
 			mb_strtolower(strip_tags($title), 'utf-8'));
 	}
 
 	/* counter cache is no longer used, if called truncate leftover data */
-	static function cleanup_counters_cache() {
+	static function cleanup_counters_cache(): void {
 		$pdo = Db::pdo();
 
 		$pdo->query("DELETE FROM ttrss_counters_cache");
 		$pdo->query("DELETE FROM ttrss_cat_counters_cache");
 	}
 
-	static function disable_failed_feeds() {
+	static function disable_failed_feeds(): void {
 		if (Config::get(Config::DAEMON_UNSUCCESSFUL_DAYS_LIMIT) > 0) {
 
 			$pdo = Db::pdo();
@@ -1599,7 +1644,7 @@ class RSSUtils {
 		}
 	}
 
-	static function housekeeping_user($owner_uid) {
+	static function housekeeping_user(int $owner_uid): void {
 		$tmph = new PluginHost();
 
 		UserHelper::load_user_plugins($owner_uid, $tmph);
@@ -1607,7 +1652,7 @@ class RSSUtils {
 		$tmph->run_hooks(PluginHost::HOOK_HOUSE_KEEPING);
 	}
 
-	static function housekeeping_common() {
+	static function housekeeping_common(): void {
 		DiskCache::expire();
 
 		self::expire_lock_files();
@@ -1623,6 +1668,9 @@ class RSSUtils {
 		PluginHost::getInstance()->run_hooks(PluginHost::HOOK_HOUSE_KEEPING);
 	}
 
+	/**
+	 * @return false|string
+	 */
 	static function update_favicon(string $site_url, int $feed) {
 		$icon_file = Config::get(Config::ICONS_DIR) . "/$feed.ico";
 
@@ -1687,11 +1735,14 @@ class RSSUtils {
 		return $icon_file;
 	}
 
-	static function is_gzipped($feed_data) {
+	static function is_gzipped(string $feed_data): bool {
 		return strpos(substr($feed_data, 0, 3),
 				"\x1f" . "\x8b" . "\x08", 0) === 0;
 	}
 
+	/**
+	 * @return array<int, array<string, mixed>> An array of filter arrays with keys "id", "match_any_rule", "inverse", "rules", and "actions"
+	 */
 	static function load_filters(int $feed_id, int $owner_uid) {
 		$filters = array();
 
@@ -1809,7 +1860,7 @@ class RSSUtils {
 	 *
 	 * @param string $url A feed or page URL
 	 * @access public
-	 * @return mixed The favicon URL, or false if none was found.
+	 * @return false|string The favicon URL string, or false if none was found.
 	 */
 	static function get_favicon_url(string $url) {
 
@@ -1843,8 +1894,12 @@ class RSSUtils {
 		return $favicon_url;
 	}
 
-	// https://community.tt-rss.org/t/problem-with-img-srcset/3519
-	static function decode_srcset($srcset) {
+	/**
+	 * @see https://community.tt-rss.org/t/problem-with-img-srcset/3519
+	 *
+	 * @return array<int, array<string, string>> An array of srcset subitem arrays with keys "url" and "size"
+	 */
+	static function decode_srcset(string $srcset): array {
 		$matches = [];
 
 		preg_match_all(
@@ -1862,7 +1917,10 @@ class RSSUtils {
 		return $matches;
 	}
 
-	static function encode_srcset($matches) {
+	/**
+	 * @param array<int, array<string, string>> $matches An array of srcset subitem arrays with keys "url" and "size"
+	 */
+	static function encode_srcset(array $matches): string {
 		$tokens = [];
 
 		foreach ($matches as $m) {
@@ -1872,7 +1930,7 @@ class RSSUtils {
 		return implode(",", $tokens);
 	}
 
-	static function function_enabled($func) {
+	static function function_enabled(string $func): bool {
 		return !in_array($func,
 						explode(',', str_replace(" ", "", ini_get('disable_functions'))));
 	}
diff --git a/classes/sanitizer.php b/classes/sanitizer.php
index 3f6e9504e..e0db811c6 100644
--- a/classes/sanitizer.php
+++ b/classes/sanitizer.php
@@ -1,6 +1,10 @@
 <?php
 class Sanitizer {
-	private static function strip_harmful_tags($doc, $allowed_elements, $disallowed_attributes) {
+	/**
+	 * @param array<int, string> $allowed_elements
+	 * @param array<int, string> $disallowed_attributes
+	 */
+	private static function strip_harmful_tags(DOMDocument $doc, array $allowed_elements, $disallowed_attributes): DOMDocument {
 		$xpath = new DOMXPath($doc);
 		$entries = $xpath->query('//*');
 
@@ -40,7 +44,7 @@ class Sanitizer {
 		return $doc;
 	}
 
-	public static function iframe_whitelisted($entry) {
+	public static function iframe_whitelisted(DOMElement $entry): bool {
 		$src = parse_url($entry->getAttribute("src"), PHP_URL_HOST);
 
 		if (!empty($src))
@@ -49,11 +53,16 @@ class Sanitizer {
 		return false;
 	}
 
-	private static function is_prefix_https() {
+	private static function is_prefix_https(): bool {
 		return parse_url(Config::get(Config::SELF_URL_PATH), PHP_URL_SCHEME) == 'https';
 	}
 
-	public static function sanitize($str, $force_remove_images = false, $owner = false, $site_url = false, $highlight_words = false, $article_id = false) {
+	/**
+	 * @param array<int, string>|null $highlight_words Words to highlight in the HTML output.
+	 *
+	 * @return false|string The HTML, or false if an error occurred.
+	 */
+	public static function sanitize(string $str, bool $force_remove_images = false, int $owner = null, string $site_url = null, array $highlight_words = null, int $article_id = null) {
 
 		if (!$owner && isset($_SESSION["uid"]))
 			$owner = $_SESSION["uid"];
@@ -183,7 +192,7 @@ class Sanitizer {
 			$div->appendChild($entry);
 		}
 
-		if ($highlight_words && is_array($highlight_words)) {
+		if (is_array($highlight_words)) {
 			foreach ($highlight_words as $word) {
 
 				// http://stackoverflow.com/questions/4081372/highlight-keywords-in-a-paragraph
diff --git a/classes/timehelper.php b/classes/timehelper.php
index 4317f343f..453ee0cee 100644
--- a/classes/timehelper.php
+++ b/classes/timehelper.php
@@ -1,7 +1,7 @@
 <?php
 class TimeHelper {
 
-	static function smart_date_time($timestamp, $tz_offset = 0, $owner_uid = false, $eta_min = false) {
+	static function smart_date_time(int $timestamp, int $tz_offset = 0, int $owner_uid = null, bool $eta_min = false): string {
 		if (!$owner_uid) $owner_uid = $_SESSION['uid'];
 
 		if ($eta_min && time() + $tz_offset - $timestamp < 3600) {
@@ -21,8 +21,8 @@ class TimeHelper {
 		}
 	}
 
-	static function make_local_datetime($timestamp, $long, $owner_uid = false,
-					$no_smart_dt = false, $eta_min = false) {
+	static function make_local_datetime(?string $timestamp, bool $long, int $owner_uid = null,
+					bool $no_smart_dt = false, bool $eta_min = false): string {
 
 		if (!$owner_uid) $owner_uid = $_SESSION['uid'];
 		if (!$timestamp) $timestamp = '1970-01-01 0:00';
@@ -67,7 +67,7 @@ class TimeHelper {
 		}
 	}
 
-	static function convert_timestamp($timestamp, $source_tz, $dest_tz) {
+	static function convert_timestamp(int $timestamp, string $source_tz, string $dest_tz): int {
 
 		try {
 			$source_tz = new DateTimeZone($source_tz);
diff --git a/classes/urlhelper.php b/classes/urlhelper.php
index 4d11b5a4d..91e1d4822 100644
--- a/classes/urlhelper.php
+++ b/classes/urlhelper.php
@@ -6,16 +6,35 @@ class UrlHelper {
 		"tel"
 	];
 
+	// TODO: class properties can be switched to PHP typing if/when the minimum PHP_VERSION is raised to 7.4.0+
+	/** @var string */
 	static $fetch_last_error;
+
+	/** @var int */
 	static $fetch_last_error_code;
+
+	/** @var string */
 	static $fetch_last_error_content;
+
+	/** @var string */
 	static $fetch_last_content_type;
+
+	/** @var string */
 	static $fetch_last_modified;
+
+	/** @var string */
 	static $fetch_effective_url;
+
+	/** @var string */
 	static $fetch_effective_ip_addr;
+
+	/** @var bool */
 	static $fetch_curl_used;
 
-	static function build_url($parts) {
+	/**
+	 * @param array<string, string|int> $parts
+	 */
+	static function build_url(array $parts): string {
 		$tmp = $parts['scheme'] . "://" . $parts['host'];
 
 		if (isset($parts['path'])) $tmp .= $parts['path'];
@@ -34,12 +53,22 @@ class UrlHelper {
 	 * @param string $owner_element Owner element tag name (i.e. "a") (optional)
 	 * @param string $owner_attribute Owner attribute (i.e. "href") (optional)
 	 *
-	 * @return string Absolute URL
+	 * @return false|string Absolute URL or false on failure (either during URL parsing or validation)
 	 */
 	public static function rewrite_relative($base_url, $rel_url, string $owner_element = "", string $owner_attribute = "") {
-
 		$rel_parts = parse_url($rel_url);
 
+		/**
+		 * If parse_url failed to parse $rel_url return false to match the current "invalid thing" behavior
+		 * of UrlHelper::validate().
+		 *
+		 * TODO: There are many places where a string return value is assumed.  We should either update those
+		 * to account for the possibility of failure, or look into updating this function's return values.
+		 */
+		if ($rel_parts === false) {
+			return false;
+		}
+
 		if (!empty($rel_parts['host']) && !empty($rel_parts['scheme'])) {
 			return self::validate($rel_url);
 
@@ -80,8 +109,10 @@ class UrlHelper {
 		}
 	}
 
-	// extended filtering involves validation for safe ports and loopback
-	static function validate($url, $extended_filtering = false) {
+	/** extended filtering involves validation for safe ports and loopback
+	 * @return false|string false if something went wrong, otherwise the URL string
+	 */
+	static function validate(string $url, bool $extended_filtering = false) {
 
 		$url = clean($url);
 
@@ -107,6 +138,11 @@ class UrlHelper {
 				} else {
 					$tokens['host'] = idn_to_ascii($tokens['host']);
 				}
+
+				// if `idn_to_ascii` failed
+				if ($tokens['host'] === false) {
+					return false;
+				}
 			}
 		}
 
@@ -138,7 +174,10 @@ class UrlHelper {
 		return $url;
 	}
 
-	static function resolve_redirects($url, $timeout, $nest = 0) {
+	/**
+	 * @return false|string
+	 */
+	static function resolve_redirects(string $url, int $timeout, int $nest = 0) {
 
 		// too many redirects
 		if ($nest > 10)
@@ -189,12 +228,16 @@ class UrlHelper {
 		return false;
 	}
 
-		// TODO: max_size currently only works for CURL transfers
+	/**
+	 * @param array<string, bool|int|string>|string $options
+	 * @return false|string false if something went wrong, otherwise string contents
+	 */
+	// TODO: max_size currently only works for CURL transfers
 	// TODO: multiple-argument way is deprecated, first parameter is a hash now
 	public static function fetch($options /* previously: 0: $url , 1: $type = false, 2: $login = false, 3: $pass = false,
 				4: $post_query = false, 5: $timeout = false, 6: $timestamp = 0, 7: $useragent = false*/) {
 
-		self::$fetch_last_error = false;
+		self::$fetch_last_error = "";
 		self::$fetch_last_error_code = -1;
 		self::$fetch_last_error_content = "";
 		self::$fetch_last_content_type = "";
@@ -510,7 +553,10 @@ class UrlHelper {
 		}
 	}
 
-	public static function url_to_youtube_vid($url) {
+	/**
+	 * @return false|string false if the provided URL didn't match expected patterns, otherwise the video ID string
+	 */
+	public static function url_to_youtube_vid(string $url) {
 		$url = str_replace("youtube.com", "youtube-nocookie.com", $url);
 
 		$regexps = [
diff --git a/classes/userhelper.php b/classes/userhelper.php
index ea714b76b..90d073d55 100644
--- a/classes/userhelper.php
+++ b/classes/userhelper.php
@@ -32,7 +32,7 @@ class UserHelper {
 	/** has administrator permissions */
 	const ACCESS_LEVEL_ADMIN			= 10;
 
-	static function authenticate(string $login = null, string $password = null, bool $check_only = false, string $service = null) {
+	static function authenticate(string $login = null, string $password = null, bool $check_only = false, string $service = null): bool {
 		if (!Config::get(Config::SINGLE_USER_MODE)) {
 			$user_id = false;
 			$auth_module = false;
@@ -99,7 +99,7 @@ class UserHelper {
 		}
 	}
 
-	static function load_user_plugins(int $owner_uid, PluginHost $pluginhost = null) {
+	static function load_user_plugins(int $owner_uid, PluginHost $pluginhost = null): void {
 
 		if (!$pluginhost) $pluginhost = PluginHost::getInstance();
 
@@ -114,7 +114,7 @@ class UserHelper {
 		}
 	}
 
-	static function login_sequence() {
+	static function login_sequence(): void {
 		$pdo = Db::pdo();
 
 		if (Config::get(Config::SINGLE_USER_MODE)) {
@@ -159,7 +159,7 @@ class UserHelper {
 		}
 	}
 
-	static function print_user_stylesheet() {
+	static function print_user_stylesheet(): void {
 		$value = get_pref(Prefs::USER_STYLESHEET);
 
 		if ($value) {
@@ -170,7 +170,7 @@ class UserHelper {
 
 	}
 
-	static function get_user_ip() {
+	static function get_user_ip(): ?string {
 		foreach (["HTTP_X_REAL_IP", "REMOTE_ADDR"] as $hdr) {
 			if (isset($_SERVER[$hdr]))
 				return $_SERVER[$hdr];
@@ -179,7 +179,7 @@ class UserHelper {
 		return null;
 	}
 
-	static function get_login_by_id(int $id) {
+	static function get_login_by_id(int $id): ?string {
 		$user = ORM::for_table('ttrss_users')
 			->find_one($id);
 
@@ -189,7 +189,7 @@ class UserHelper {
 			return null;
 	}
 
-	static function find_user_by_login(string $login) {
+	static function find_user_by_login(string $login): ?int {
 		$user = ORM::for_table('ttrss_users')
 			->where('login', $login)
 			->find_one();
@@ -200,7 +200,7 @@ class UserHelper {
 			return null;
 	}
 
-	static function logout() {
+	static function logout(): void {
 		if (session_status() === PHP_SESSION_ACTIVE)
 			session_destroy();
 
@@ -211,11 +211,11 @@ class UserHelper {
 		session_commit();
 	}
 
-	static function get_salt() {
+	static function get_salt(): string {
 		return substr(bin2hex(get_random_bytes(125)), 0, 250);
 	}
 
-	static function reset_password($uid, $format_output = false, $new_password = "") {
+	static function reset_password(int $uid, bool $format_output = false, string $new_password = ""): void {
 
 		$user = ORM::for_table('ttrss_users')->find_one($uid);
 		$message = "";
@@ -298,7 +298,7 @@ class UserHelper {
 		}
 	}
 
-	static function get_otp_secret(int $owner_uid, bool $show_if_enabled = false) {
+	static function get_otp_secret(int $owner_uid, bool $show_if_enabled = false): ?string {
 		$user = ORM::for_table('ttrss_users')->find_one($owner_uid);
 
 		if ($user) {
@@ -333,7 +333,9 @@ class UserHelper {
 		return null;
 	}
 
-	static function is_default_password() {
+	static function is_default_password(): bool {
+
+		/** @var Auth_Internal|false $authenticator -- this is only here to make check_password() visible to static analyzer */
 		$authenticator = PluginHost::getInstance()->get_plugin($_SESSION["auth_module"]);
 
 		if ($authenticator &&
@@ -345,10 +347,12 @@ class UserHelper {
 		return false;
 	}
 
-	static function hash_password(string $pass, string $salt, string $algo = "") {
-
-		if (!$algo) $algo = self::HASH_ALGOS[0];
-
+	/**
+	 * @param string $algo should be one of UserHelper::HASH_ALGO_*
+	 *
+	 * @return false|string False if the password couldn't be hashed, otherwise the hash string.
+	 */
+	static function hash_password(string $pass, string $salt, string $algo = self::HASH_ALGOS[0]) {
 		$pass_hash = "";
 
 		switch ($algo) {