It sounds like you do not have Oracle support compiled in PHP.
Create a page with only a phpinfo tag ( "<?php phpinfo() ?>" and check to see that the "Configure Command" section contains the "--with-oci8" directive.
You should also have an "oci8" subsection later in the page.
I'm guessing you have neither. If this is the case, you'll need to recompile PHP with the '--with-oci8' directive. You MUST have an installed Oracle client on the system to do this...PHP uses the libraries from Oracle.
Jim