根据sessionID判断用户是否登陆

login.php

<?php
session_start();

$user = $_GET["user"];
$session_ID = session_id();
$_SESSION[$session_ID]=$user;

print $session_ID;

登陆后返回sessionID

http://localhost/login.php?user=root

用户执行一些需要已经在登陆状态的操作

带上sessionID

http://localhost/islogin.php?sessionID=a3dms2f84b55i2pg9r2gffm0v5

我们可以通过sessionID来判断是哪个用户

islogin.php

<?php
session_start();
$session_id = $_GET['sessionID'];
if (isset($_SESSION[$session_id])){
    print $_SESSION[$session_id] ;

} else{
    print ("未登陆");
}

用户注销时也注销sessionID

loginout.php

<?php

session_start();

session_id = _GET['sessionID'];

session_unset($session_id);

访问loginout后

http://localhost/loginout.php?sessionID=a3dms2f84b55i2pg9r2gffm0v5

再次判断是否登陆

http://localhost/islogin.php?sessionID=a3dms2f84b55i2pg9r2gffm0v5