I was wondering if it was possible to place some php code on each and every page of my content on my site to detect the number of visitors on the site at any one moment just like what PHPNuke is able to do.

Eg.

Page 1 --> *php code -> 2 visitors
Page 2 --> *php code -> 6 visitors
Page 3 --> *php code -> 4 visitors

Secret url (e.g http://www.mydomain.com/visitors.php) -> 12 visitors

Any way to do that?