Another one for posterity here. I was asked to find out how to upgrade on PHP RHEL 5 / CentOS 5 to v 5.3.x and to test the procedure. It turns out to work pretty well and is not as difficult as you might think as long as you have the right repositories enabled:
rpm -Uvh epel-release-5-4.noarch.rpm
rpm -Uvh remi-release-5.rpm
yum –enablerepo=remi update php php-* mysql
This, of course, assumes that your LAMP stack is already installed. If not, you would change the “update” to “install” and away you go. This will currently set you to php v 5.3.3 and mysql 5.1.51..