Add a configuration check for encoding detection

Author: Kevinjil

webapp/src/Service/CheckConfigService.php

@@ -189,6 +189,12 @@ public function checkPhpSettings(): ConfigCheckItem
                     (isset($sizes[$var]) ? Utils::printsize($sizes[$var]) : "unlimited"));
         }
 
+        $desc .= "\nFor submission encoding detection, the detect order should at least contain all expected submission encodings:\n";
+        $desc .= sprintf("  - `mbstring.detect_order` should at least contain `UTF-8` (now set to `%s`)\n", implode(', ', mb_detect_order()));
+        if ($result !== 'E' && !in_array('UTF-8', mb_detect_order())) {
+            $result = 'W';
+        }
+
         $this->stopwatch->stop(__FUNCTION__);
         return new ConfigCheckItem(
             caption: 'PHP settings',