In this quick tutorial on the Bollysite blog showing how to upgrade your CentOS install of PHP from 5.1 to 5.3 along with xcache.

CentOS 5 comes with php 5.1 version. There is no official PHP 5.2+ release for upgrade since last 3 years. [...] As a result, Developer had to implement alternative functions to integrate twitter, myspace OAuth API. Today wordpress has officially said bye bye to php 4 and mysql 4. So finally, I gathered some courage to mess with my current php installation. I followed the following steps to upgrade php 5.3 on centos 5.

The update requires adding the webtatic repository to your yum setup and using the "enablerepo" to pull directly from that. Once that's installed, you'll need to rebuild xcache to match the version (commands included).