private static function make_request($endpoint, $values, $headers = array())
    {
        self::$ch = curl_init();
        $headers = array_merge(array("Content-Type: application/json"), $headers); // always pass the correct Content-Type header

        // send Api Version header
        if (!empty(self::$api_version)) {
            $headers[] = "Api-Version: " . self::$api_version;
        }

        curl_setopt(self::$ch, CURLOPT_USERAGENT, 'WePay v2 PHP SDK v' . self::VERSION . ' Client id:' . self::$client_id);
        curl_setopt(self::$ch, CURLOPT_RETURNTRANSFER, true);
        curl_setopt(self::$ch, CURLOPT_HTTPHEADER, $headers);
        curl_setopt(self::$ch, CURLOPT_TIMEOUT, 30); // 30-second timeout, adjust to taste
        curl_setopt(self::$ch, CURLOPT_POST, !empty($values)); // WePay's API is not strictly RESTful, so all requests are sent as POST unless there are no request values

        // Force TLS 1.2 connections
        curl_setopt(self::$ch, CURLOPT_SSL_VERIFYPEER, true);
        curl_setopt(self::$ch, CURLOPT_SSL_VERIFYHOST, 2);
        curl_setopt(self::$ch, CURLOPT_SSLVERSION, CURL_SSLVERSION_TLSv1_2);

        $uri = self::getDomain() . $endpoint;
        curl_setopt(self::$ch, CURLOPT_URL, $uri);

        if (!empty($values)) {
            curl_setopt(self::$ch, CURLOPT_POSTFIELDS, json_encode($values));
        }

        $raw = curl_exec(self::$ch);
        if ($errno = curl_errno(self::$ch)) {
            // Set up special handling for request timeouts
            if ($errno == CURLE_OPERATION_TIMEOUTED) {
                throw new WePayServerException("Timeout occurred while trying to connect to WePay");
            }
            throw new Exception('cURL error while making API call to WePay: cURL Errno - ' . $errno . ', ' . curl_error(self::$ch), $errno);
        }

        $result = json_decode($raw);
        $httpCode = curl_getinfo(self::$ch, CURLINFO_HTTP_CODE);

        if ($httpCode >= 400) {
            if (!isset($result->error_code)) {
                throw new WePayServerException("WePay returned an error response with no error_code, please alert api@wepay.com. Original message: $result->error_description", $httpCode, $result, 0);
            }
            if ($httpCode >= 500) {
                throw new WePayServerException($result->error_description, $httpCode, $result, $result->error_code);
            }
            switch ($result->error) {
                case 'invalid_request':
                    throw new WePayRequestException($result->error_description, $httpCode, $result, $result->error_code);
                case 'access_denied':
                default:
                    throw new WePayPermissionException($result->error_description, $httpCode, $result, $result->error_code);
            }
        }

        return $result;
    }