API

    /**
     * @param array $data
     * @param string $secret
     * @return string
     */
    public static function generateSignature(array $data, string $secret): string
    {
        unset($data['sign']);
        ksort($data);
        foreach ($data as $key => $val) {
            if ($val === '' || is_array($val)) {
                unset($data[$key]);
            }
        }
        return md5(urldecode(http_build_query($data) . "&key=" . (string)$secret));
    }