Authenticating via http

Here's a really quick question. I can't figure out how to try to get user authentication more than once. For instance, supposed I get this log on screen that asks for my username and password. If I type in something wrong that variable gets stuck in $PHP_AUTH_USER, and there is no way to unset it, so the user would be prompted again. Please help. Thanks <?php if (!isset($PHP_AUTH_USER)) { header("WWW-Authenticate: Basic realm=\"My Realm\""); header("HTTP/1.0 401 Unauthorized"); echo "Text to send if user hits Cancel button\n"; exit; } else { //Check the database for user/password combination. Oh, not right? let's try again unset($PHP_AUTH_USER); //Doesn't work } ?>