PHP keep login details


I’m doing this site where a user logs in with a username and password… My problem is that as soon as the user closes the browser, (tested on FF and IE) his login details are lost and has to log in again next time he visits the site.
(if the user leaves the site, keeps the same browser window open and then goes back to the site, the login is still there)

so i’m thinking that when the browser closes it’s deleting the session variables. any idea how this can be solved?

my sessions are created:

$cache_expire = session_cache_expire();
ini_set("session.gc_maxlifetime", "18000"); 

header("Cache-control: private");