Several applications need more modern PHP release like 5.2 or 5.3 to run on CentOS 5.
As the generic codebase does not get updated you need to add a repository and update from there.
rpm --import http://repo.webtatic.com/yum/RPM-GPG-KEY-webtatic-andy wget -P /etc/yum.repos.d/ http://repo.webtatic.com/yum/webtatic.repo
yum --enablerepo=webtatic update php