User exits logout operation

<?php /** * Created by PhpStorm. * User: Administrator * Date: 2018/2/27 0027 * Time: 上午 10:47 */ header('Content-type:text/html;charset=utf-8'); if ($_SERVER['REQUEST_METHOD'] == 'POST') { if (empty($_POST['username'])){ echo "<script>alert('用户名不能为空！');location.href='login.html';</script>"; }else { $username=$_POST['username']; } if (empty($_POST['password'])){ echo "<script>alert('密码不能为空！');location.href='login.html';</script>"; }else{ $password=$_POST['password']; } } $mysqli = new mysqli('localhost', 'root', 'root', 'student'); $result = $mysqli->query("SELECT password FROM user WHERE username = "."'$username'"); $rs=$result->fetch_row(); if (!empty($rs)){ if ($password != $rs[0]) { echo "<script>alert('密码错误！');location.href='login.html';</script>"; }else{ $expire=3600; ini_set('session.gc_maxlifetime', $expire);//保存1小时 if (empty($_COOKIE['PHPSESSID'])) { session_set_cookie_params($expire); session_start(); }else{ session_start(); setcookie('PHPSESSID', session_id(), time() + $expire); } if(isset($_SESSION['username'])){ exit("您已经登入了，请不要重新登入！用户名：{$_SESSION['username']}---<a href='logout.php'>注销</a>"); }else{ $_SESSION['username']=$_POST['username']; } echo "<script>alert('登录成功！');</script><br>"; echo "您好！{$_SESSION['username']},欢迎回来！"; echo "<a href='logout.php'>注销</a>"; } }else{ echo "<script>alert('没有此用户！');location.href='login.html';</script>"; }