The current defalut build/install of PHP has register globals turned off. So if you have a form with a entry field named email, you can't access it as $email in a fresh build of PHP 4.2.1. You'd have to access $POST["email"], or $GET["email"].
Or you can change your php.ini file to allow the conversion of input data to local variables (this is a security risk).
http://www.php.net/manual/en/configuration.php#ini.register-globals