Hmmm, first thing I'd do is call phpinfo() with the long (<?php) tag; not that it'd do much good, I expect...
Are you using name-based VirtualHosts, or IP based? Is PHP a standalone module, or the Apache DSO?
Sounds likes a hard look at httpd.conf might be in order; try and determine whether the AddModule, LoadModule, and AddType directives are in a location such that they do not apply to the entire server, or are we facing a different issue?