Remi Collet Repository - (See http://rpms.famillecollet.com/) Has been recommended on the mailing list and forum for mysql 5.1 and php5.2.9. Repo configs for EL5 i386 and x86_64 as well as EL4 are available. Suggest extreme caution if using this repo.

http://blog.famillecollet.com/pages/Config-en

ну и гуглим на предмет php 5.2.9 centos remi

wget http://download.fedora.redhat.com/pub/epel/5/i386/epel-release-5-2.noarch.rpm
wget http://rpms.famillecollet.com/el5.i386/remi-release-5-4.el5.remi.noarch.rpm
rpm -Uvh remi-release-5*.rpm epel-release-5*.rpm
yum –enablerepo=remi update php

Для возврата к старой версии PHP
yum –disablerepo=remi
yum remove php php-cli php-common
yum install php