PHP - Expirando a sessão por tempo

<?php

//Start our session.
session_start();

//Expire the session if user is inactive for 20 minutes or more.
$expireAfter = 20;

//Check to see if our "last action" session variable has been set.
if(isset($_SESSION['last_action'])){

    //Figure out how many seconds have passed since the user was last active.
    $secondsInactive = time() - $_SESSION['last_action'];

    //Convert our minutes into seconds.
    $expireAfterSeconds = $expireAfter * 60;

    //Check to see if they have been inactive for too long.
    if($secondsInactive >= $expireAfterSeconds){
        //User has been inactive for too long. Kill their session.
        session_unset();
        session_destroy();
    }
}

//Assign the current timestamp as the user's latest activity
$_SESSION['last_action'] = time();

?>