code: protected function composeFields($id = null, $data = null)
      {
          $fields = $this->writeCallback ? call_user_func($this->writeCallback, $this) : [];
          if ($id !== null) {
              $fields['id'] = $id;
          }
          if ($data !== null) {
              $fields['data'] = $data;
          }
          return $fields;
      }
docs: Composes storage field set for session writing.
  @param string $id Optional session id
  @param string $data Optional session data
  @return array storage fields