Add a new session variable and save to order?

I did some search but was unable to find the answer:

I want to add a session variable, and save it to the order. What should be the best way to do that? (assume I can add a new column to the order table for it)

For example, I may use this session variable to save the referral URL to my page.

How do I add a session variable? What are the files I should change? And which file is responsible for saving an order.

Thanks a lot.

For example, I may use this session variable to save the referral URL to my page.
Don't mess with sessions
There are a couple of affiliate 3rd party modules in the free software addons, link above left, that can bu used

kobra:

Don't mess with sessions
There are a couple of affiliate 3rd party modules in the free software addons, link above left, that can bu used

Well, it doen't mean I can't try to learn how to make a better one. :-)

Well, it doen't mean I can't try to learn how to make a better one.
The existing ones would be a good place to begin