Sessions that last for ever

By default, a session (created with session_register) seems to last just as long as the user has their browser open. If a user quits the browser, the session is automatically destroyed. I want a session to last indefinitely (or until my program destroys it). There are some hints about how this could be done with cookies in the documentation, but not a clear recipe. What sequence of PHP statements should I use to achieve this? Thanks for any help, Nigel