From http://snipplr.com/view.php?codeview&id=72803

how_to_expire_a_php_session_after_x_minutes.php

if (isset($_SESSION['LAST_ACTIVITY']) && (time() - $_SESSION['LAST_ACTIVITY'] > 1800)) {
    // last request was more than 30 minutes ago
    session_unset();     // unset $_SESSION variable for the run-time 
    session_destroy();   // destroy session data in storage
}
$_SESSION['LAST_ACTIVITY'] = time(); // update last activity time stamp

/*
You can also use an additional time stamp to regenerate the session ID periodically to avoid attacks on sessions like session fixation:
*/
if (!isset($_SESSION['CREATED'])) {
    $_SESSION['CREATED'] = time();
} else if (time() - $_SESSION['CREATED'] > 1800) {
    // session started more than 30 minutes ago
    session_regenerate_id(true);    // change session ID for the current session an invalidate old session ID
    $_SESSION['CREATED'] = time();  // update creation time
}

//note that session.gc_maxlifetime should be at least equal to the life time of this custom expiration handler (1800 in this example).