The good folk at Les RPM de Remi have compiled some rpms allowing you to add oci8 to your Enterprise Linux php environment.
The repo main page lists the available mirrors (use a mirror, not the main repo).
Here’s the link to php-oci8 for EL6 x86_64. Those posted now are for php 5.3.8 and 5.3.9.
Since RHEL 6 and its clones ship with php 5.3.3 you’ll need to upgrade in order to use these. My usual source for this is the Rackspace’s IUS Repo.
Note: In setting up my latest SL 6.1 workstation I opted to stick with the 5.3.3 version of php and build php-oci8 from the pecl source. The latest configure script works fine as long as you remember to specify your instantclient location and have php-devel installed (the phpize command is installed with devel).