<?
// $expire = the time in seconds until a session have to expire
function start_session($expire = 0) {
    if ($expire == 0) {
        $expire = ini_get("session.gc_maxlifetime");
    } else {
        ini_set("session.gc_maxlifetime", $expire);
    }
    if (empty($_COOKIE[‘PHPSESSID’])) {
        session_set_cookie_params($expire);
        session_start();
    } else {
        session_start();
        setcookie("PHPSESSID", session_id(), time() + $expire);
    }
}
// this example will start a session with an expire time given by the php configuration
start_session();
// start_session(600) will start a session which will expire after 10 minutes (60*10 seconds)
?>

Just memo, this is source.