enigma1 - 12:31 pm on Jan 9, 2012 (gmt 0)
The 'HTTP_REFERER' is not a trusted variable. So it can be manipulated, don't rely on it. Doing curls or fsockopens will add latency to your server. If they want to manipulate it and still give out a fake hard-coded link they can do so in the page you try to parse.
var fakelink = '<a href="http://www.example.com">www.example.com</a>';
Spiders may not treat it as a link but you will. Now you gonna need more than a simple parser to detect it. And there are other ways to fake it.