Problem with unset($PHP_AUTH_USER );

I use a login script with the header() function and $PHP_AUTH_USER etc. the problem is once i've logged in once ,i refresh the page i stay logged in. i want to do testing with different privileges so i want to be asked for a username and password each time i've refreshed the page.I thought unset($PHP_AUTH_USER) right after i check the db would work but it doesn't.I've also tried varieties of ways to avoid caching the page as well but nothing seems to work.this i know is a damn simple problem. but it's driving me a bit nuts.i even tried emptying my temp internet files but to no avail. what's going on? thanx a