PHP跨域会话

I seem to be having a problem with cross-domain sessions. I have set the session name on both (sub) domains and it is still not working. When I set the session I also output $_SESSION and username is set on sub1.x.com but when I try to access it on sub2.x.com it does not output. What am I doing wrong here?

Here's what I am doing:

session_name('gbixtld'); ini_set('session.cookie_domain', '.gbizit.uk'); ini_set('session.save_path', '/var/lib/php/session'); session_start();