just setup a new vps with centos and needed the latest php5.3 - heres the shortcuts

sudo rpm –import http://repo.webtatic.com/yum/RPM-GPG-KEY-webtatic-andy
cd /etc/yum.repos.d/
sudo wget http://repo.webtatic.com/yum/webtatic.repo
sudo yum –enablerepo=webtatic update php

php-xml is not availble in the webtatic repo - you can manually download the rpm from fubra with wget

sudo wget http://mirror.fubra.com/php-5.3-repo/5/i386/php-xml-5.3.0-3.i386.rpm
sudo rpm -ivh php-xml-5.3.0-3.i386.rpm

restart apache, check you php.ini and away you go

—
refer: webtatic blog post for more info