Originally Posted by StuntPope

I may be wrong on all this:

HTTP_REFERER is typically an address, an actual URI of some referring document. A bookmark stored by a browser interface isn't a URI (it isn't an address/location on the web, it's just a link in a config file, and when I say this I don't include bookmarking sites like del.icio.us, whose bookmarks *would* be an HTTP_REFERER when clicked)

The best you can do in this case is maybe is REMOTE_ADDR or REMOTE_HOST, but then a lot of mobile devices would be connecting over NAT through a proxy, so you'd wind up with the IP/URL of the proxy.