Is PHP 5.3.3 on RHEL 6.5 receiving security fixes?

Solution 1:

Yes, Red Hat backports security fixes and important bug fixes to all packages they offer in their repositories until the EOL of that particular RHEL version. New major features or drastic changes are not appearing in RHEL updates.

Solution 2:

If you are using the PHP that is included in the official Redhat packages, then it is supported. They go out of their way to avoid the - 'want a security fix, have these incompatible changes as well' problem. Looking at the the Redhat version number and comparing with upstream is unhelpful for security issues.

As it happens, the latest update for php53 was issued today.