public function validate($url)
    {
        if (null !== ($headers = $this->getHeader($url))) {
            $statusCode = $this->getStatusCode($headers);
            // see https://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
            return ($statusCode >= 200 && $statusCode < 300);
        }
        return false;
    }