My guess is register globals is off, and the other server had it on.
What that means is if you're grabbing variables from a form, on your other server, you could call it by using $varname
If register globals is off, you'd need to call it by using $_POST['varname']
There are other predefined variables you'd need to change to have your site work also, but the best thing to do if you don't want to go through and change everything is in your php.ini file, set register globals to on.
Cgraz