Using Server Variables

[qwertykey]

I am trying to get my php script to read a certain part of the url in the browser. When I attempt to do
$_REQUEST["id"]
it does not work, I get an Undefined variable. So my question is: What function/command can I use to get part of a url from the browser? Could I maybe see an example?

Thanks

[bbaassiri]

$_GET['id']