look at php.ini and change this line:
"register_globals = Off" to "register_globals = On"
but that can be bad idea... for multi-reasons, so do not set it "On" and if you want get variables from url you must write (as your example):
so if you want get variables from URL use $_GET['var'] and from form $_POST['var'] and cookie $_COOKIE['var'] and server as $PHP_SELF must be $_SERVER['$PHP_SELF'].
there more as $_SESSION, $_REQUEST, $_ENV, $_FILES and $GLOBALS... just check http://php.net/variables