/**
     * @desc 生成指定随机字符串
     * @param $length
     * @return string|null
     * @date 2021/4/23
     */
    function get_rand_char($length)
    {
        $str = null;
        $strPol = "ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
        $max = strlen($strPol) - 1;

        for ($i = 0; $i < $length; $i++) {
            $str .= $strPol[rand(0, $max)];
        }

        return $str;
    }