private function generateMACSignature($url, $parameters, $http_method)
    {
        $timestamp = time();
        $nonce = uniqid();
        $parsed_url = parse_url($url);
        if (!isset($parsed_url['port']))
        {
            $parsed_url['port'] = ($parsed_url['scheme'] == 'https') ? 443 : 80;
        }
        if ($http_method == self::HTTP_METHOD_GET) {
            if (is_array($parameters)) {
                $parsed_url['path'] .= '?' . http_build_query($parameters, null, '&');
            } elseif ($parameters) {
                $parsed_url['path'] .= '?' . $parameters;
            }
        }

        $signature = base64_encode(hash_hmac($this->access_token_algorithm,
                    $timestamp . "\n"
                    . $nonce . "\n"
                    . $http_method . "\n"
                    . $parsed_url['path'] . "\n"
                    . $parsed_url['host'] . "\n"
                    . $parsed_url['port'] . "\n\n"
                    , $this->access_token_secret, true));

        return 'id="' . $this->access_token . '", ts="' . $timestamp . '", nonce="' . $nonce . '", mac="' . $signature . '"';
    }