Session dies but cookie is still there

[Anon]

I have a login system for multilple users, my program uses sessions with cookies after a certain amount of time the session seems to die and so the user can no longer access the page as i have this code:
if (session_is_registered("valid_user"))
where valid user is set upon login, refreshing the page and closing the browser does not kill the session i'm not sure what's happening cus after a while (not sure the time) the 'valid_user' is not available anymore.
The cookie seems to live still though! when I open the cookie it has the correct variable values for the current 'valid_user'.
Can somone help?
thanks

[Anon]

this is my php.ini file settings

session.auto_start Off (i use session_star())
session.cache_expire 180
session.cache_limiter nocache
session.cookie_domain no value
session.cookie_lifetime 86400
session.cookie_path /
session.cookie_secure Off
session.entropy_file no value
session.entropy_length 0
session.gc_maxlifetime 86400
session.gc_probability 0
session.name PHPSESSID
session.referer_check no value
session.save_handler files
session.save_path C:\Documents and Settings\andrew_b\project1\tmp
session.serialize_handler php
session.use_cookies On

The sessions seem to die for no reason but the cookie is still there containing the correct information