<?
if(로그인 성공){
    $_SESSION["auth"] = "super";
    $_SESSION['login_time_stamp'] = time();
}
?>

<?php
if( !empty($_SESSION["auth"])){
    if (time()-$_SESSION["login_time_stamp"] > 60 * 30) {  //로그인 지속시간(분단위, 30분)
        unset($_SESSION["auth"]);
        session_unset(); 
        session_destroy();
    } else {
        $_SESSION['login_time_stamp'] = time();
    }
}
?>