I found this on another website
// GET vars don't work, only POST.
if (!$HTTP_POST_VARS || $HTTP_GET_VARS) {
echo( "No vars were posted to the form. Check your form design and make sure
you set the \"method\" attribute on the form tag to \"post\"; i.e;
<form action=\"receipt.php3\" method=\"post\">. You should not see
any form information in the URL if everything is set up right.");
}