public static function arrayToXml($arr, $num_prefix = "num_") : string
    {
        if (!is_array($arr)) return $arr;
        $result = '';
        foreach ($arr as $key => $val) {
            $key = (is_numeric($key) ? $num_prefix . $key : $key);

            $result .= '<' . $key . '>' . self::arrayToXml($val, $num_prefix) . '</' . $key . '>';
        }
        return $result;
    }