How info is passed to $vars

I know that if you call a page like this ... http://www.domain.com/index.php?thisvar=1&thatvar=2 ... that you get $thisvar and $thatvar to use in your script, but exactly what conditions are necessary for a "POST" and "GET" to be passed as variables into the PHP engine. I presume that there is a header that flags an incoming result as a post or get? The reason I ask is that I am receiving a kind of form "POST" that is failing to register the $vars in the usual way, and I presume that this is because they are using a different header. I know that this company will not change their script (for reasons too boring to explain) and so I would like to know how I can get PHP accept special header info, is it apache that is responsible in this case or PHP ? I understand that the information is the same format as a "POST" but that the header is a little different. Zim