A lot of sites have "page.php" but it also has a ?id= or just a ? after it. After the ? theres usually a bunch of numbers so it ends up looking like page.php?id=23569 or something similar. Can someone explain to me what's happening there?
With page.php?id=something the page.php script would be able to access the $_GET['id'] field to determine that it has a value of 'something' and then use that value to control what appears on the page.
Sure it's possible, depending on whether you can specify the precise rule(s) that determines what part of the value is to be used. For instance, if you want to skip the first 4 characters of whatever the value is:
$value = substr($_GET['id'], 4);
"Please give us a simple answer, so that we don't have to think, because if we think, we might find answers that don't fit the way we want the world to be."
~ Terry Pratchett in Nation
You would need to determine what the rule is for what part of the value you want before being able to work out the best way of extracting that part probably either by using a substr or regular expression.
It is always preferable security-wise to reference $_GET when reading the querystring because with $_REQUEST you don't know whether you have obtained the value from a GET, a POST, or a COOKIE.