I think that you have hit the issue on the nose. Sorry for wasting your time on this. But, I have one last question. How do you un-associate ANY file extension in vista. This feature seems to have been overlooked by the Microsoft developers.
Sorry, this is more like a Windows question, but there might not be any way to unregister file extensions. If that .php extension was registered by another program, uninstalling that program might unregister that file extension. If not, I would go to RegEdit.exe, locate HKEY_CLASSES_ROOT.php, and delete that tree or a portion of that tree or values. Please make sure you backup the registry information before you edit any values or keys in the Registry, so you can undo you changes if there was something wrong.