Yes, you can use HTTP_REFERER to do this. It's really the only way, except for asking the users directly. And it is very easily faked or blocked. Still, the majority of users know not how to do this, so you can probably grab a lot this way. Parsing your access logs would be the best way to grab this, of course. Finally, make sure that your server is actually *logging* this. It usually appears as the last field in the access_log for apache servers.