And I may have misunderstood fcarsenal's post with register_globals off you can still pass variables from an HTML form but you have to access them with the correct global array like $_POST or $_GET or $_REQUEST - the link I posted details this.
Ask your host WHY? If he can't give you a good answer, it could be because he has no idea what he is doing. Correct me if I am wrong here but I think it's safe to say that most hosts around here have it turned off.
when php 4.3 came out the php group turned it off by default. unfortunately, when it is left on it can be a security vulnerability because you can pass variables to the page that are not supposed to be accesable from the URL such as something that the page would get from a cookie or a session variable. The downside, is that any URLs passwd through the URL need to be defined in the code now before they can be used but using the $HTTP_POST_VARS array (and similar arrays). the php manual should have all the info you need to modify your script to work.