Idle times

[FatesCall]

Sorry for posting so much this is probably going to be my last post. I have my sessions working and pages restricted now thanks to you guys but I have a few more questions.

1) How would I make it to where if the user is inactive for longer than 30 minutes it logs them out and forces them to log in again? 
2) In my logout php I would just use exit() and session_destroy(), correct? 

EDIT: 

So I wrote this but I'm not sure if it will work:

if (isset($_SESSION['LAST_ACTIVITY']) && (time() - $_SESSION['LAST_ACTIVITY'] > 1800)) {
    session_unset();
    session_destroy();   
}
$_SESSION['LAST_ACTIVITY'] = time(); // Call this every time a user submits/clicks on something

[jcbones]

You don't need to use exit(), unless you want the script to stop running. Otherwise, you are on the right track.