Upgrading to php 5.2 in Centos 5.x

To enable centos-dev, you need to create /etc/yum.repos.d/centos-test.repo and add the following

[c5-testing]
name=CentOS-5 Testing
baseurl=http://dev.centos.org/centos/5/testing/$basearch/
enabled=1
gpgcheck=1
gpgkey=http://dev.centos.org/centos/RPM-GPG-KEY-CentOS-testing

Then when you

yum update php

You will get the latest version.  Please note it breaks ioncube and you will need to download the latest version from http://www.ioncube.com/loaders.php

Thanks to this comment for showing the way.