Affiliate system

Hi guys,

I am reviewing an affiliate system that I created a while ago. I am using a
very simple method to do this, but I am curious if there is any better
system (better I mean less missed affiliate purchases).

1. User click on affiliate link http://thedomain.com/?a=1234
2. The if $_GET['a'] is set, we check if that is an valid affiliate, and
then set a cookie for that
3. At purchase, we check if that cookie still is there, if so, we register
it as a affiliate purchase and that info is stored in the purchase database

What should I do if they do not allow cookies? I could amend a=1234 onto
every url that they pass, but that would not be a "beautiful" solution.

Maybe I can also create a SESSION that stores the affiliate information so
that it at least get registered if the purchase is completed within the
session?

What do you think is the best way?

Best regards,
Peter Lauri

www.lauri.se - personal web site
www.dwsasia.com - company web site
.