这个我还没有完全搞明白，记录一下，方便以后查找。

<html>
<head>
    <title>用户登录</title>
</head>
<body>
    <form name="form1" action="28.php" method="post">
        <table border="0" cellspacing="0" cellpadding="">
            <tr>
                <td>用户名：</td>
                <td><input type="text" name="user_name" id="user_name"></td>
            </tr>
            <tr>
                <td>密码：</td>
                <td><input type="password" name="password" id="password"></td>
            </tr>
            <tr>
                <td colspan="2"><input type="submit" name="button1" id="button1" value="提 交"></td>
            </tr>
        </table>
    </form>
</body>
</html>
<?php
/**
 * Created by PhpStorm.
 */

$users = array(
    array('user_name'=>'tom','password'=>'1','style'=>'css1'),
    array('user_name'=>'jake','password'=>'2','style'=>'css2'),
    array('user_name'=>'seven','password'=>'3','style'=>'css3'),
    array('user_name'=>'andy','password'=>'4','style'=>'css4'),
    array('user_name'=>'king','password'=>'5','style'=>'css5'),
    array('user_name'=>'robert','password'=>'6','style'=>'css6'),
);

function is_login()
{
    global $users;

    $u = $_COOKIE['user_name'];
    $p = $_COOKIE['password'];
    foreach($users as $key => $value)
    {
        if($value['user_name'] == $u && $value['password'] == $p)
        {
            return true;
        }
    }

    return false;
}

function login()
{
    if(is_login())
    {
        echo "<div class='css'>";
        echo "你好：".$_COOKIE['user_name']."&nbsp;&nbsp;&nbsp;&nbsp;<a href='?action=logout'>注销</a>";
        echo "</div>";
    }
    else
    {
        global $users;

        $u = $_POST['user_name'];
        $p = $_POST['password'];
        foreach ($users as $key=>$value)
        {
            if($value['user_name'] == $u && $value['password'] == $p)
            {
                setcookie('user_name',$value['user_name']);
                setcookie('password',$value['password']);
                setcookie('style',$value['style']);
                echo "<script>alert('登录成功！');</script>";
                return true;
            }
        }

        echo "<script>alert('用户名或密码错误！');window.history.go(-1);</script>";
        return false;
    }

}

function logout()
{
    setcookie('user_name',"");
    setcookie('password','');
    echo "<script>alert('注销成功！');</script>";
    header("location:27.php");
}

if(isset($_GET['action']))
{
    switch ($_GET['action'])
    {
        case "login":
            login();
            break;
        case "logout":
            logout();
            break;
    }
}
else
{
    login();
}