I think what he's getting at is the use of register_globals, which is a PHP configuration setting that, if enabled, turns parameters passed to a PHP script into variables of the same name. On Dreamhost, if you're using PHP5, which I assume you are, this setting is disabled.
What you want to do is to check $GET for the parameters instead. In short, the way to do this is to replace all references to $rnd, $name, $email, etc. with $GET['rnd'], $GET['name'], $GET['email'], etc. You will also want to delete this line completely:
global $name, $email, $subject, $body;