Basic Auth

I"m using basic auth for a few of my pages that I want to limit access to - nothing of a sensitive nature, but simply want to limit access to. Want to implement a signoff process, but can't figure it out. From the comments in the manual I take it one can't do this by simply unsetting the PHP_AUTH_USER and _PW vars. Can someone explain to me why this doesn't suffice? The signon process expects them to be there, so when they are not (after the 'unset'), how come my signon process still detects them and their values?