I would like a script, written in JavaScript, that passes a variable to the server in a url string, and then sets the variable as a cookie on the visitors computer. I would then like to be able to retrieve this cookie and pass it as a hidden form field, when a particular form is submitted.

Example:
http://www.mydomain.com/?afid=123456

Once the visitor gets to www.mydomain.com , the afid is read; a cookie is set with the value 123456, on the visitors computer.

Then when an order form is submitted, I would like to have a hidden form field that I can set and have it pass the cookie as part of the form submission.

This is very similar to an affiliate program where a affiliate sends their unique link, and it sets the value that can be retrieved to determine the source of the order later.

Thanks