PHP implements simple login function (source code attached)

<form action="login.php" method="post">
<fieldset>
<legend>用户登录</legend>
<ul>
<li>
<label>用户名:</label>
<input type="text" name="username">
</li>
<li>
<label>密   码:</label>
<input type="password" name="password">
</li>
<li>
<label> </label>
<input type="checkbox" name="remember" value="yes">7天内自动登录
</li>
<li>
<label> </label>
<input type="submit" name="login" value="登录">
</li>
</ul>
</fieldset>
</form>

<?php 
header(&#39;Content-type:text/html; charset=utf-8&#39;);
// 开启Session
session_start();
 
// 处理用户登录信息
if (isset($_POST[&#39;login&#39;])) {
# 接收用户的登录信息
$username = trim($_POST[&#39;username&#39;]);
$password = trim($_POST[&#39;password&#39;]);
// 判断提交的登录信息
if (($username == &#39;&#39;) || ($password == &#39;&#39;)) {
// 若为空,视为未填写,提示错误,并3秒后返回登录界面
header(&#39;refresh:3; url=login.html&#39;);
echo "用户名或密码不能为空,系统将在3秒后跳转到登录界面,请重新填写登录信息!";
exit;
} elseif (($username != &#39;username&#39;) || ($password != &#39;password&#39;)) {
# 用户名或密码错误,同空的处理方式
header(&#39;refresh:3; url=login.html&#39;);
echo "用户名或密码错误,系统将在3秒后跳转到登录界面,请重新填写登录信息!";
exit;
} elseif (($username = &#39;username&#39;) && ($password = &#39;password&#39;)) {
# 用户名和密码都正确,将用户信息存到Session中
$_SESSION[&#39;username&#39;] = $username;
$_SESSION[&#39;islogin&#39;] = 1;
// 若勾选7天内自动登录,则将其保存到Cookie并设置保留7天
if ($_POST[&#39;remember&#39;] == "yes") {
setcookie(&#39;username&#39;, $username, time()+7*24*60*60);
setcookie(&#39;code&#39;, md5($username.md5($password)), time()+7*24*60*60);
} else {
// 没有勾选则删除Cookie
setcookie(&#39;username&#39;, &#39;&#39;, time()-999);
setcookie(&#39;code&#39;, &#39;&#39;, time()-999);
}
// 处理完附加项后跳转到登录成功的首页
header(&#39;location:index.php&#39;);
}
}
 ?>

<?php 
header(&#39;Content-type:text/html; charset=utf-8&#39;);
// 开启Session
session_start();
 
// 首先判断Cookie是否有记住了用户信息
if (isset($_COOKIE[&#39;username&#39;])) {
# 若记住了用户信息,则直接传给Session
$_SESSION[&#39;username&#39;] = $_COOKIE[&#39;username&#39;];
$_SESSION[&#39;islogin&#39;] = 1;
}
if (isset($_SESSION[&#39;islogin&#39;])) {
// 若已经登录
echo "你好! ".$_SESSION[&#39;username&#39;].&#39; ,欢迎来到个人中心!<br>&#39;;
echo "<a href=&#39;logout.php&#39;>注销</a>";
} else {
// 若没有登录
echo "您还没有登录,请<a href=&#39;login.html&#39;>登录</a>";
}
 ?>

<?php 
header(&#39;Content-type:text/html; charset=utf-8&#39;);
// 注销后的操作
session_start();
// 清除Session
$username = $_SESSION[&#39;username&#39;];  //用于后面的提示信息
$_SESSION = array();
session_destroy();
 
// 清除Cookie
setcookie(&#39;username&#39;, &#39;&#39;, time()-99);
setcookie(&#39;code&#39;, &#39;&#39;, time()-99);
 
// 提示信息
echo "欢迎下次光临, ".$username.&#39;<br>&#39;;
echo "<a href=&#39;login.html&#39;>重新登录</a>";
 
 ?>