Drupal 6: How to determine if User is Logged In or not

In Drupal, data for the current user is stored in the global variable $user. To determine if the user is logged in, use the following code:

<?php
    global $user;
    if ($user->uid) {
        // User is logged in.
    }
?>

This works because all guests browsing the site will have a uid of 0. That will return FALSE in the IF statement while any positive number will return TRUE. In Drupal, logged in users are referred to as authenticated users.