HTTP Authentication and PHP

Hi all, I've been playing around with PHP authentication via HTTP. I'm using apache, and when i use the header('WWW_Auth...) headers i get a username/password dialog pop up (as i wanted). How do i get those values unset in the browser, so that i can get a user to re authenticate ? I need to get a user to re-auth part way through the use of the app i'm writing becasue they are deleting files from a file system. Also how would i allow them to logout and let someone else log in. The docs seemed to imply that the headers command should have repopped the auth box. Jason