To install, first you must tell rpm to accept rpm’s signed by me, then add the yum repository information to yum:
rpm -ivh http://repo.webtatic.com/yum/centos/5/`uname -i`/webtatic-release-5-1.noarch.rpm

Now you can install php by doing:
yum --enablerepo=webtatic install php

Or update an existing installation of php, which will also update all of the other php modules installed:
yum --enablerepo=webtatic update php

If this does not work correctly, try disabling all other repositories while installing/updating, by adding the –disablerepo=* option. This will stop other dependencies from being installed, so you may want to install them first.
yum --disablerepo=* --enablerepo=webtatic update php