Just to be honest, this PHP 5.3 upgrade/update has been made painless by the GREAT folks at IUS Community Repo! The real magic is their repository setup and rather helpful "yum-plugin-replace" yum plugin. I have not seen any problem with this update path at all. My needs may be different than yours, so YMMV. This post is adapted from a previous post that focused on another program that just needed a PHP > 5.2 installed.
You must be the root user to execute all the stated commands.
rpm -Uhv ius-release-1.0-8.ius.el5.noarch.rpm epel-release-5-4.noarch.rpm
yum install yum-plugin-replace
yum replace php --replace-with php53
yum search php53
service httpd restart
service mysqld restart
service postgresql restart