So the thing is we don't really handle the PHP packages. Mostly because InterWorx is not super dependent on the version of PHP that runs on the server. As long as it's a set of PHP packages built and distributed for the version of RHEL or CentOS you are using, it should integrate just fine with Apache and InterWorx.
My quick recommendation is to grab something like Atomic Turtle and install that. BEFORE running yum update or yum install anything, refer to this article:
FAQ: Which packages should I exclude from 3rd-party repos?
and exclude packages that need to come from the InterWorx repository in the /etc/yum.repos.d/*.repo file that corresponds to atomic turtle. Alternatively, you can leave the repository disabled, and do a:
everytime you want to update PHP. This will localize the use of the repository to just when you want to update PHP.
yum --enablerepo=atomic update php