I originally posted on the Drupal Support Forum here.
Here is how I did it in Drupal 5.11
Hope this helps someone. I created a block, then used Panels to add this block to my Panel page. Don’t forget to add the PHP opening and closing tags.
$result = db_fetch_array(db_query("select count(uid) from users where week(from_unixtime(created)) = week(date_sub(now(), interval 1 week))")); print $result['count(uid)'];