$HTTP_RAW_POST_DATA isn't set unless PHP registers globals. By default, PHP 5 does not register globals.
A quick fix is adding the following to googleNotify.php before the first use of $HTTP_RAW_POST_DATA:
$HTTP_RAW_POST_DATA = file_get_contents('php://input');
I would also name it something other than $HTTP_RAW_POST_DATA to avoid confusion with the global variables of PHP's past.
(It's possible – but unlikely – my notification issues were caused by a different problem, because I fixed several things at once.)