First of all, make sure you have register_globals=on
in your php.ini file.
If that wasn't it, you may be suffering from the problem that has been reported by a couple of people in the past.
We still don't know for sure what the actual cause is, but from what I've seen on the PHP bug tracker, it may even be a bug in PHP itself. I would be intested to hear if an upgrade to PHP 4.3.0 fixes this (if it wasn't the register_globals setting).
bye, Dirk