diff --git a/application/Utils.php b/application/Utils.php
index 073004cf..48d47415 100644
--- a/application/Utils.php
+++ b/application/Utils.php
@@ -61,6 +61,7 @@ function smallHash($text)
  */
 function startsWith($haystack, $needle, $case = true)
 {
+    $needle = $needle ?? '';
     if ($case) {
         return (strcmp(substr($haystack, 0, strlen($needle)), $needle) === 0);
     }
@@ -180,7 +181,7 @@ function generateLocation($referer, $host, $loopTerms = [])
         $host = substr($host, 0, $pos);
     }
 
-    $refererHost = parse_url($referer, PHP_URL_HOST);
+    $refererHost = parse_url($referer, PHP_URL_HOST) ?? '';
     if (!empty($referer) && (strpos($refererHost, $host) !== false || startsWith('?', $refererHost))) {
         $finalReferer = $referer;
     }
@@ -292,7 +293,7 @@ function generate_api_secret($username, $salt)
  */
 function normalize_spaces($string)
 {
-    return preg_replace('/\s{2,}/', ' ', trim($string));
+    return preg_replace('/\s{2,}/', ' ', trim($string ?? ''));
 }
 
 /**
diff --git a/application/api/controllers/HistoryController.php b/application/api/controllers/HistoryController.php
index d83a3a25..e16036f6 100644
--- a/application/api/controllers/HistoryController.php
+++ b/application/api/controllers/HistoryController.php
@@ -30,7 +30,7 @@ class HistoryController extends ApiController
         $history = $this->history->getHistory();
 
         // Return history operations from the {offset}th, starting from {since}.
-        $since = \DateTime::createFromFormat(\DateTime::ATOM, $request->getParam('since'));
+        $since = \DateTime::createFromFormat(\DateTime::ATOM, $request->getParam('since', ''));
         $offset = $request->getParam('offset');
         if (empty($offset)) {
             $offset = 0;
diff --git a/application/bookmark/BookmarkArray.php b/application/bookmark/BookmarkArray.php
index b9328116..0c1a6eca 100644
--- a/application/bookmark/BookmarkArray.php
+++ b/application/bookmark/BookmarkArray.php
@@ -57,7 +57,7 @@ class BookmarkArray implements \Iterator, \Countable, \ArrayAccess
      *
      * @return int Number of bookmarks
      */
-    public function count()
+    public function count(): int
     {
         return count($this->bookmarks);
     }
@@ -70,7 +70,7 @@ class BookmarkArray implements \Iterator, \Countable, \ArrayAccess
      *
      * @throws InvalidBookmarkException
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         if (
             ! $value instanceof Bookmark
@@ -106,7 +106,7 @@ class BookmarkArray implements \Iterator, \Countable, \ArrayAccess
      *
      * @return bool true if it exists, false otherwise
      */
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return array_key_exists($this->getBookmarkOffset($offset), $this->bookmarks);
     }
@@ -116,7 +116,7 @@ class BookmarkArray implements \Iterator, \Countable, \ArrayAccess
      *
      * @param int $offset Bookmark ID
      */
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         $realOffset = $this->getBookmarkOffset($offset);
         $url = $this->bookmarks[$realOffset]->getUrl();
@@ -132,7 +132,7 @@ class BookmarkArray implements \Iterator, \Countable, \ArrayAccess
      *
      * @return Bookmark|null The Bookmark if found, null otherwise
      */
-    public function offsetGet($offset)
+    public function offsetGet($offset): ?Bookmark
     {
         $realOffset = $this->getBookmarkOffset($offset);
         return isset($this->bookmarks[$realOffset]) ? $this->bookmarks[$realOffset] : null;
@@ -143,7 +143,7 @@ class BookmarkArray implements \Iterator, \Countable, \ArrayAccess
      *
      * @return Bookmark corresponding to the current position
      */
-    public function current()
+    public function current(): Bookmark
     {
         return $this[$this->keys[$this->position]];
     }
@@ -153,7 +153,7 @@ class BookmarkArray implements \Iterator, \Countable, \ArrayAccess
      *
      * @return int Bookmark ID corresponding to the current position
      */
-    public function key()
+    public function key(): int
     {
         return $this->keys[$this->position];
     }
@@ -161,7 +161,7 @@ class BookmarkArray implements \Iterator, \Countable, \ArrayAccess
     /**
      * Iterator - Moves forward to next element
      */
-    public function next()
+    public function next(): void
     {
         ++$this->position;
     }
@@ -171,7 +171,7 @@ class BookmarkArray implements \Iterator, \Countable, \ArrayAccess
      *
      * Entries are sorted by date (latest first)
      */
-    public function rewind()
+    public function rewind(): void
     {
         $this->keys = array_keys($this->ids);
         $this->position = 0;
@@ -182,7 +182,7 @@ class BookmarkArray implements \Iterator, \Countable, \ArrayAccess
      *
      * @return bool true if the current Bookmark ID exists, false otherwise
      */
-    public function valid()
+    public function valid(): bool
     {
         return isset($this->keys[$this->position]);
     }
diff --git a/application/bookmark/LinkUtils.php b/application/bookmark/LinkUtils.php
index 8fa2953a..112b6f00 100644
--- a/application/bookmark/LinkUtils.php
+++ b/application/bookmark/LinkUtils.php
@@ -219,7 +219,7 @@ function tags_str2array(?string $tags, string $separator): array
     // For whitespaces, we use the special \s regex character
     $separator = $separator === ' ' ? '\s' : $separator;
 
-    return preg_split('/\s*' . $separator . '+\s*/', trim($tags) ?? '', -1, PREG_SPLIT_NO_EMPTY);
+    return preg_split('/\s*' . $separator . '+\s*/', trim($tags ?? ''), -1, PREG_SPLIT_NO_EMPTY);
 }
 
 /**
diff --git a/application/formatter/BookmarkDefaultFormatter.php b/application/formatter/BookmarkDefaultFormatter.php
index 91985ee1..70f65818 100644
--- a/application/formatter/BookmarkDefaultFormatter.php
+++ b/application/formatter/BookmarkDefaultFormatter.php
@@ -120,7 +120,7 @@ class BookmarkDefaultFormatter extends BookmarkFormatter
                 $prefix = rtrim($this->contextData['base_path'], '/') . '/';
             }
 
-            return escape($prefix ?? '') . escape(ltrim($bookmark->getUrl(), '/'));
+            return escape($prefix ?? '') . escape(ltrim($bookmark->getUrl() ?? '', '/'));
         }
 
         return escape($bookmark->getUrl());
diff --git a/application/front/controller/admin/ThumbnailsController.php b/application/front/controller/admin/ThumbnailsController.php
index 5dfea096..665dfd03 100644
--- a/application/front/controller/admin/ThumbnailsController.php
+++ b/application/front/controller/admin/ThumbnailsController.php
@@ -45,7 +45,7 @@ class ThumbnailsController extends ShaarliAdminController
      */
     public function ajaxUpdate(Request $request, Response $response, array $args): Response
     {
-        $id = $args['id'] ?? null;
+        $id = $args['id'] ?? '';
 
         if (false === ctype_digit($id)) {
             return $response->withStatus(400);
diff --git a/application/helper/ApplicationUtils.php b/application/helper/ApplicationUtils.php
index f79998b5..063300ca 100644
--- a/application/helper/ApplicationUtils.php
+++ b/application/helper/ApplicationUtils.php
@@ -238,7 +238,7 @@ class ApplicationUtils
                  $conf->get('resource.update_check'),
              ] as $path
         ) {
-            if (!is_file(realpath($path))) {
+            if (!is_string($path) || !is_file(realpath($path))) {
                 # the file may not exist yet
                 continue;
             }
diff --git a/application/http/Url.php b/application/http/Url.php
index fe87088f..129957bf 100644
--- a/application/http/Url.php
+++ b/application/http/Url.php
@@ -61,6 +61,7 @@ class Url
      */
     public function __construct($url)
     {
+        $url = $url ?? '';
         $url = self::cleanupUnparsedUrl(trim($url));
         $this->parts = parse_url($url);
 
diff --git a/application/legacy/LegacyLinkDB.php b/application/legacy/LegacyLinkDB.php
index d3beafe0..cb19eda5 100644
--- a/application/legacy/LegacyLinkDB.php
+++ b/application/legacy/LegacyLinkDB.php
@@ -116,7 +116,7 @@ class LegacyLinkDB implements Iterator, Countable, ArrayAccess
     /**
      * Countable - Counts elements of an object
      */
-    public function count()
+    public function count(): int
     {
         return count($this->links);
     }
@@ -124,7 +124,7 @@ class LegacyLinkDB implements Iterator, Countable, ArrayAccess
     /**
      * ArrayAccess - Assigns a value to the specified offset
      */
-    public function offsetSet($offset, $value)
+    public function offsetSet($offset, $value): void
     {
         // TODO: use exceptions instead of "die"
         if (!$this->loggedIn) {
@@ -155,7 +155,7 @@ class LegacyLinkDB implements Iterator, Countable, ArrayAccess
     /**
      * ArrayAccess - Whether or not an offset exists
      */
-    public function offsetExists($offset)
+    public function offsetExists($offset): bool
     {
         return array_key_exists($this->getLinkOffset($offset), $this->links);
     }
@@ -163,7 +163,7 @@ class LegacyLinkDB implements Iterator, Countable, ArrayAccess
     /**
      * ArrayAccess - Unsets an offset
      */
-    public function offsetUnset($offset)
+    public function offsetUnset($offset): void
     {
         if (!$this->loggedIn) {
             // TODO: raise an exception
@@ -179,7 +179,7 @@ class LegacyLinkDB implements Iterator, Countable, ArrayAccess
     /**
      * ArrayAccess - Returns the value at specified offset
      */
-    public function offsetGet($offset)
+    public function offsetGet($offset): ?array
     {
         $realOffset = $this->getLinkOffset($offset);
         return isset($this->links[$realOffset]) ? $this->links[$realOffset] : null;
@@ -188,14 +188,17 @@ class LegacyLinkDB implements Iterator, Countable, ArrayAccess
     /**
      * Iterator - Returns the current element
      */
-    public function current()
+    public function current(): array
     {
         return $this[$this->keys[$this->position]];
     }
 
     /**
      * Iterator - Returns the key of the current element
+     *
+     * @return int|string
      */
+    #[\ReturnTypeWillChange]
     public function key()
     {
         return $this->keys[$this->position];
@@ -204,7 +207,7 @@ class LegacyLinkDB implements Iterator, Countable, ArrayAccess
     /**
      * Iterator - Moves forward to next element
      */
-    public function next()
+    public function next(): void
     {
         ++$this->position;
     }
@@ -214,7 +217,7 @@ class LegacyLinkDB implements Iterator, Countable, ArrayAccess
      *
      * Entries are sorted by date (latest first)
      */
-    public function rewind()
+    public function rewind(): void
     {
         $this->keys = array_keys($this->ids);
         $this->position = 0;
@@ -223,7 +226,7 @@ class LegacyLinkDB implements Iterator, Countable, ArrayAccess
     /**
      * Iterator - Checks if current position is valid
      */
-    public function valid()
+    public function valid(): bool
     {
         return isset($this->keys[$this->position]);
     }