Is there a way of doing a simple visitor count, ideally by incrementing a number in a text file... eg: http://mysite.org/?ref=client1 would increment a number in text file 'client1', and still display http://mysite.org as normal?
If you use mod_rewrite, you could have all requests redirect to a PHP script (or any other script), which would increase the txt file counter and then pass the request through to the page you want. Here is a link to a simple counter script: http://www.stevedawson.com/scripts/text-counter.php