Okay so I'm not using any session variables, rather my code looks like this:

if (!isset($_SERVER['PHP_AUTH_USER'])) {
    header('WWW-Authenticate: Basic realm="Enter your Twitter username and password:"');
    header('HTTP/1.0 401 Unauthorized');
    echo 'Please enter your Twitter username and password to view your followers.';
    exit();
}

$username = $_SERVER['PHP_AUTH_USER'];
$password = $_SERVER['PHP_AUTH_PW'];

So, my question is, how can I destroy this login session when the user wants to sign out of their (in this case) twitter login credentials?