Most likely, register_globals has been turned off. They are turned off for security reasons. As a quick fix, you can bypass this by adding this line to the top of your PHP script:
foreach ($_REQUEST as $k => $v) { $$k = $v; }
Of course, by adding that line, you are essentially saying that you don't want the security protection that comes from turning register_globals off so keep that in mind before you use this command. If you are concerned about it, search PHPBuilder.com or Google for register_globals and find out why it's turned off by default.