How to implement simple password login in php

<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>';
		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>';
	echo "<a href=&#39;login.html&#39;>重新登录</a>";

 ?>