public function checkHash() {
    $string = '';
    $hashFieldName = $this->requestMethod->getNotifyHashName();

    foreach ($this->notifyParams as $k => $v) {
      if ($k !== $hashFieldName) {
        $string .= $v;
      }
    }

    if ($this->notifyParams[$hashFieldName] === hash_hmac('md5', $string, $this->secret)) {
      return TRUE;
    }

    return FALSE;
  }