The Linux Fix yum repository has been updated with new rpm’s for the tlf-release package and for php version 5.3.6. The new tlf-release package includes changes to the layout of the yum repository, including splitting off a separate 32-bit and 64-bit repo. You can read the release notes for php 5.3.6 here.
You may install or update the TLF repository with the following command:
rpm -Uvh http://yum.thelinuxfix.com/RHEL/5/i386/tlf-repo-1-2.noarch.rpm
Note that this command will work on 64-bit architectures as well. I also need to repeat the disclaimer that using any of the database extensions for this version of php will also update sqlite to version 3.6.20, which replaces some pretty core functionality of RHEL and CentOS, so please use them at your own risk.