Use a keep alive.
On login:
session_start();
$_SESSION['last_action'] = time();
An ajax call every few (eg 20) seconds:
windows.setInterval(keepAliveCall, 20000);
Server side keepalive.php:
session_start();
$_SESSION['last_action'] = time();
On every other action:
session_start();
if ($_SESSION['last_action'] < time() - 30 /* be a little tolerant here */) {
// destroy the session and quit
}