Have you tried checking phpinfo() yet?
Create a php file named info.php, then copy paste the following into it. Place it into your web directory and navigate to it from your browser. I'm hoping you know how to do this. And by navigating to it from your browser I don't mean File->open file. I mean http://localhost/info.php
if you happen to put it in your root web server directory. Make sure apache (httpd) is running. Let me know if you have any questions. The output you'll see in your browser will tell you if php was compiled with oci8 support
If you do not have php compiled with oci8 support (yes I know you installed php via yum) this means you'll need to uninstall php from your system and download the source code and compile it with oci8 support. Unless of course you can find a package on the net with this already compiled in. You'll also need the oracle client installed as you'll need to point php to your oracle installation directory. Lots of stuff to do and know when adding oracle support.