session cookie expire jump thing

I’m using sessions for the first time, and I need to make it so that when the session expires in 30 minutes, it jumps to the login page. How would i go about doing this?

my session looks like this atm

[code=php]<?php
/* set the cache limiter to ‘private’ */

session_cache_limiter(‘private’);
$cache_limiter = session_cache_limiter();

/* set the cache expire to 30 minutes */
session_cache_expire(30);
$cache_expire = session_cache_expire();

session_start();
if(!isset($_SESSION[username])){
header(“Location:index.php?func=login”);
}
?>[/code]

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