It's register_globals. You probably won't be allowed to change php.ini, but it might be possible to use httpd.conf to set it for each directory. (How? It's in the manual. Hint: somewhere under [man]configuration[/man]).
Really, though, you have only your own laziness to blame for your scripts not working anymore: $GET and $POST have been available since December 2001, and register_globals has been off by default since April last year. After all the posts on the subject here you can hardly say you weren't warned.