Add c5-testing repository to yum:
cd /etc/yum.repos.d/ wget http://dev.centos.org/centos/5/CentOS-Testing.repo
You can then view the latest available php packages with:
yum --disablerepo=\* --enablerepo=c5-testing list \*php\*
Then you can install them with:
yum --disablerepo=\* --enablerepo=c5-testing install php
