function getrandstr(){
        $str='ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890';
        $randStr = str_shuffle($str);//打乱字符串
        $rands= substr($randStr,0,6);//substr(string,start,length);返回字符串的一部分
        return $rands;
    }