This is awfully light on the details, but I'll take a shot at it anyway.
My guess is that your code depends on register_globals to be enabled and that your older sites are using PHP4, which has it enabled, and the new one is PHP5, which has it disabled. Read about register_globals at the Dreamhost support wiki for a little more information.
If this isn't the case, follow up with some details, particularly the code you're using and the URLs you're using it at.
If you want useful replies, ask smart questions.