Well, as an overview, your /index.php would have the code

$pubid = $_REQUEST['pubid'];

to capture the affiliate ID. Are you also looking for other information to capture? Then if you want to log this ID by saving it in a database,

$result = mysql_query("INSERT INTO tablename (affiliate_ID) VALUES ('$pubid');");

would log every single occurrence. (Of course, that isn't the full code, but just the heart of it.) My SQL book isn't at hand, but you might be able to add a keyword such as UNIQUE to prevent duplicates. Or, you could first do a query to see if that ID is already in the database, and INSERT it only if it's not. Presumably you'd want to save more information than just the affiliate ID, such as a timestamp or a count of hits or the incoming IP address.