Fixes #11666 - Second try.

extension/Classes/Core/Helper/Sanitize.php

@@ -7,7 +7,6 @@
  */
 
 namespace IMATHUZH\Qfq\Core\Helper;
- 
 
 
 /**
@@ -289,14 +288,19 @@ class Sanitize {
      */
     public static function digitCheckAndCleanGet($key) {
 
-        if (isset($_GET[$key]) && !ctype_digit($_GET[$key])) {
-            if (!empty($_GET[$key]) && is_array($_GET[$key]) && ctype_digit($_GET[$key][0])) {
-                $_GET[$key] = $_GET[$key][0];
-            } else {
-                $_GET[$key] = '';
-            }
+        if (!isset($_GET[$key])) {
+            $_GET[$key] = '';
+            return;
         }
 
-    }
+        if (ctype_digit($_GET[$key])) {
+            return;
+        }
 
+        if (ctype_digit($_GET[$key][0] ?? '')) {
+            $_GET[$key] = $_GET[$key][0];
+        } else {
+            $_GET[$key] = '';
+        }
+    }
 }
\ No newline at end of file

extension/Tests/Unit/Core/Helper/SanitizeTest.php

@@ -349,7 +349,7 @@ class SanitizeTest extends TestCase {
 
         unset ($_GET[CLIENT_PAGE_LANGUAGE]);
         Sanitize::digitCheckAndCleanGet(CLIENT_PAGE_LANGUAGE);
-        $this->assertEquals(false, isset($_GET[CLIENT_PAGE_LANGUAGE]));
+        $this->assertEquals('', $_GET[CLIENT_PAGE_LANGUAGE]);
 
         $_GET[CLIENT_PAGE_LANGUAGE] = '';
         Sanitize::digitCheckAndCleanGet(CLIENT_PAGE_LANGUAGE);