关于XSS漏洞代码修复

提交大作业，老师说这里面有XSS漏洞，求教一下漏洞在哪？怎么改？非常感谢<?php/** * Created by PhpStorm. * User: dell1 * Date: 2016/7/27 * Time: 22:59 */$a="<br/>";    $check=$_POST["check"];    session_start();    $user = $_SESSION['user'];    $check_mobile=$_SESSION['rand'];    $ps=$_SESSION['password'];    $mobile=$_SESSION['mobile'];    if ($check_mobile != $check) {        exit('please input the right verification code!<a href="javascript:history.back(-1);">back');    }    else{        //连接数据库        $mysql=new mysqli('127.0.0.1','root','123','user');        if (mysqli_connect_errno($mysql))        {   echo 'ERROR: could not connect the database';            exit(1);        }        mysqli_query("set names 'utf8'");//检查是否存在相同用户名        $check=mysqli_query($mysql,"select users from users where users='$user'");        $result=mysqli_num_rows($check);        if($result)        {            exit('the username exists!<a href="javascript:history.back(-1);">back');        }        else        {//对用户口令哈希后连同手机号用户名一起存储            $ps_hs = password_hash($ps, PASSWORD_DEFAULT);            $mysql_insert = "insert into users(users,password,mobile)values('$user','$ps_hs','$mobile')";            if (!mysqli_query($mysql, $mysql_insert))            {                die('ERROR:' . mysqli_error($mysql));            } else            {                echo "THE USER $user REGISTER SUCCESSFULLY!'";                echo $a;            }        }    }    session_destroy();echo $a;echo('Ha ha! Welcome to be a member of my system though there is nothing!');echo $a;echo "<input type=\"button\" onclick=\"window.location.href='../html/index.html'\" value=\"return to the home page\">";