$pages = 0;
	if(session_is_registered($_SESSION['pages'])) {
		$pages = $_SESSION['pages'];
	}
	$pages++;
	$return .= "Dit is pagina $pages in deze sessie (".session_id().").<br />";
	$_SESSION['pages'] = $pages;

This keeps printing out "Dit is pagina 1 in deze sessie" with a constant session id.