private function getRequestLimitHeader()
  {
    $headers = array();
    $client  = $this->getClient();

    if ( ! is_null($client)) {
      $headers['X-Rate-Limit-Limit']   = $client->request_limit;
      $headers['X-Rate-Limit-Remaining'] = $client->request_limit - $client->current_total_request;
      $headers['X-Rate-Limit-Reset']   = strtotime($client->request_limit_until) - time();
    }

    unset($client);

    return $headers;
  }