I've never used php with oracle, but I'll try to help.
You can run:
php -i|grep configure
to see if php was compiled with oci8 support. If not (most likely), you should d/l the source code, matching your version of php, from http://www.php.net/
and compile it by adding "--with-oci8" or something like "--with-oci8=instantclient,/path/to/oic/lib" to the output of the above command. I.e. you should run something like:
./configure --with-options-from-above-command --with-oci8
Then run "make", "make install".
I guess you have to uninstall the php you're now using before running "make install", but keep the rpm handy in case something goes wrong.