Few questions about PHP sessions

I have a few a few question about php sessions:

1. 
   

   
   Since the default value for session.gc_maxlifetime is 24 mins then that means any session file that isn't modified for 24 mins will be deleted and the session will expire automatically.
   

   
   

$_SESSION created but theres no PHPSESSID in $_SERVER

I'm experiencing some weird problems with SESSION variables on my PHP/Ajax online shopping cart.

Changed cookie domain, but old cookie is still used

EDITED, look at the end

I got a Symfony 1.2 project, that was running on two domains (different app used on each domain) : www.mywebsite.com and abonnement.mywebsite.com