session判断页面是否已经登录的问题

session_start();

isset(
SESSION['username'])?include("checklogin.php"):session_destroy();

?>

欢迎页面，说明已经登录进来

<?php

if (isset(GET['out'])){
GET['out']=='index'?session_destroy():'';

}

?>

<a href='/go.html?url=?out=index'>退出</a>

checklogin.php页面是<?php

//session_start();session_start();

isset(
SESSION['username'])?i