public static function getUserAgent()
    {
        $curlVersion = curl_version(); // Do not move this inside $headers even if it is used only there.
                                       // PHP < 5.4 doesn't support call()['value'] directly.
        $userAgent = self::formatUserAgentProduct('PayPlug-PHP',
                                                  Payplug\Core\Config::LIBRARY_VERSION,
                                                  sprintf('PHP/%s; curl/%s', phpversion(), $curlVersion['version']));
        foreach (self::$defaultUserAgentProducts as $product) {
            $userAgent .= ' ' . self::formatUserAgentProduct($product[0], $product[1], $product[2]);
        }
        return $userAgent;
    }