If you set global variables to OFF, the variables sent to the next page by the browser will cannot be accesed like this as a normal variable. They must be accesed via $_POST, $_GET, $_COOKIE or $_SESSION. variables are only set in there "own" array and not in another.
allthough it should be possible for a user to "post" a variable of his own while you didn't in the form.
$_REQUEST is a global array wich contains all the above
see also this manual page, wich should explain it as well: