Hmmm... I know you can track referring websites via $_SERVER['HTTP_REFERER'], however I want to know if it's possible to track other referrers, such as emails or links in IRC rooms, IM messenger windows, etc, etc? Not using $_SERVER['HTTP_REFERER'], obviously, but with some other function?
True, then you could sift through your logfiles for that specific querystring, but what if the person removes it? I was thinking of a way that'd always be able to catch of those non-site referrers...
Let's face it, relixx, you can't definitely track web site referrers, so it's no worse than the situation that you are talking about. That is to say, just because someone followed a link to your site from another does not mean that you will see a valid referrer value anyway...
Use subdomains that forward to the real domain. eg xyz.yoursite.com forwards to www.yoursite.com via a tracking script. People are much more likely to type a subdomain than yoursite.com/xyz
Depending on what sort of content you're serving up, you could throw it subtly into the URL. If you're talking about a blog for instance, the url could be served as http://www.myblog.com/posts/3445/5/the-internet-is-tubes/, where 3445 is a randomly generated number that gets tacked on via something like a redirect or from the link itself. Looking through the logs you'd easily be able to see where people have pasted the link to someone else, via two people accessing the same random number within a short period of time.