I am a bit hesitant to suggest this, but Oracle has already built RHEL-compatible php packages, including php-OCI8. You can find the files at http://oss.oracle.com/projects/php/, built for both RHEL 4u6 and RHEL 5u1. The php-oci8 package depend on the Oracle Instant Client (also available as RPMs), which means you don't have to do a full Oracle install unless you need the actual database server.
I have used them under CentOS 5u2 with good results, but be warned, they will be NOT supported by Red Hat, Oracle, or CentOS.