:: install php 5.3 on centos 5.5

:: add php 5.3 webtatic repository

# rpm -Uvh http://repo.webtatic.com/yum/centos/5/latest.rpm

:: installing php 5.3

# yum --enablerepo=webtatic install php

or
:: update php 53

# yum --enablerepo=webtatic update php

:: links
+ google
+ webtatic