To install PHP 5.2.5 which is the latest version in the repos you can make use of a RPM repository maintained by Remi. He has a repository for many distros. Follow the steps to get it done ..
rpm -Uvh remi-release-5*.rpm epel-release-5*.rpm
Now you have remi’s repo in ur system. so to use the remis repo to install your packages you have to tell yum to use the new repo..
yum --enablerepo=remi update php
You are done..
Hope it helps