Tracking a user

I just want to make sure I have all the options down (for a php3 site): I want to be able to track a user coming from SiteA to my site and see if that user makes some sort of purchase. To do this I need to have SiteA either pass me some sort of variable and probably link to some "starting page" for this type of thing. From there, I either need to: * Set up a variable to be passed to each page on the site, to be used on every link on the site, to have when the user completes a purchase * Set up a cookie with the same information that would be/was used in the above GET variable and use the information stored in the cookie to have when the user completes a purchase. (on a side note, I looked at the setcookie() function and it doesn't appear to return a value as to whether or not the cookie actually got set. Is there any way to check to see if setting the cookie was successful other than checking for the value on the subsequent pages?) Other than the above, is there any other way that anyone knows of to track a user in a php3 environ? Thanks! Chris