PHP authentication problem

you mean want to logout? Try this:

header('WWW-Authenticate: Basic realm="This Realm"');
header('HTTP/1.0 401 Unauthorized‘);
// if a session was running, clear and destroy it
session_start();
session_unset();
session_destroy();

You can find more examples in the original PHP site
http://hk2.php.net/features.http-auth

http://www.phpdc.com/article_list/2/

Try to see this website:
http://www.phpdc.com/article_list/2/
It has a lot of teaching source and coding example,
maybe you can find the solution from this website.
Hope you lucky!
圖片參考：http://hk.yimg.com/i/icon/16/1.gif