Trying to retrieve/confirm referrer URL in a PHP page/script

As subject line says, it would be nice, sometimes, specifically in terms of form submission processing to maybe confirm the referring URL/server, and I have used similar things in past with classic ASP etc., but can't remember that exact syntax, but think it had something to do with something like a HTTP response referer header or something. Either way, I know there are collections of values like $_REQUEST, $_SERVER, etc., but not sure where to try find something like this, or if it's simply/easily possible/doable? TIA Jacob Kruger Blind Biker Skype: BlindZA '...fate had broken his body, but not his spirit...'