session ID does not delete itself

Hi, I have been experimenting with PHP4 using sessions and one of my books says that session ID's are created in the /tmp directory so I take a look in there and I find are about 10 sessions that have not been deleted dated earliest to about being week old ie. sess_5b30ccebb1d098c37a5e46efd7708fef I have been experimenting with user authentication with sessions and just plain starting a session when a user accesses the site. Well, the site is still experimental and each time I logged myself out. But I thought sessions were supposed to wipe themselves out from the /tmp directory immediately after leaving the website. Appreciate if someone could shed the light on this issue for me. Thanks. Peter