code: protected function mergeFlashMessages($messageArray)
      {
          $messages = array_values($messageArray);
          $flashMessage = '';
          $flashMessageArr = [];
          foreach ($messages as $message) {
              if (is_array($message)) {
                  if (strlen($flashMessage) > 0) {
                      $flashMessage .= '<br/>';
                  }
                  $flashMessage .= $this->mergeFlashMessages($message);
              } else {
                  $flashMessageArr[] = $message;
              }
          }
  
          return $flashMessage . implode('<br/>', $flashMessageArr);
      }
docs: merge flash messages
  @author Adeyemi Olaoye <yemi@cottacush.com>
  @param $messageArray
  @return string