随机生成四码长度的英文

    //生成随机码，默认四码，约七百次会出现碰撞
    function get_random_key($length = 4)
    {
        $permitted_chars = '1234567890abcdefghijklmnopqrstuvwxyxABCDEFGHIJKLMNOPQRSTUVWXYX';
        return substr(str_shuffle($permitted_chars), 0, $length);
    }