Thanks, Jose, for your response...I haven't gotten it quite yet, however.
Here's what I've done...
I added this into config.php
include('http://mysite.com/phpincludes/active_users.class.php');
$au = new activeUsers;
In page_header.php, I added this at the very bottom. I think there may be a syntax error at the end?
$site_template->register_vars(array("u_online" => $au-> numActive(),));
Then, in footer.html, I added the variable
{u_online}
Then I tried it and it freaked out. Something like this pops up
Fatal error: Cannot instantiate non-existent class: activeusers in /home/www/mysite/gallery/config.php on line 27
Where am I going wrong?