Adding variable to a session from external link?

[ecroskey]

I am starting to set up banner advertising online through different media outlets. I want eacj of the banners to have a unique identifier attached to their links. Once someone clicks the banner and comes to the site I want to track that identifier throughout the visitors time and if they order, write out the identifier out with the order so I know which advertising is working, not working etc.

 

So, how would I do this?

 

I was thinking about adding it to the session with session_register(code) or something but not quite sure how to this within OSC.

 

Can anyone advise?