code: public function checkFileValidity($filename)
      {
          try {
              $phar = new \Phar($filename);
              // free the variable to unlock the file
              unset($phar);
          } catch (\Exception $e) {
              if (!$e instanceof \UnexpectedValueException && !$e instanceof \PharException) {
                  throw $e;
              }
  
              return 'Failed to create Phar instance. '.$e->getMessage();
          }
  
          return null;
      }
docs: {@inheritdoc}