Certain newer packages like php 5.2 (actually this is not new) is still not available in the latest version of centos. If you are working with zend framework, you know you need the latest php version. What can you do? Install the dev repo.
create the file /etc/yum.repos.d/centos-test.repo
then append these lines to it
[c5-testing] name=CentOS-5 Testing baseurl=http://dev.centos.org/centos/5/testing/$basearch/ enabled=1 gpgcheck=1 gpgkey=http://dev.centos.org/centos/RPM-GPG-KEY-CentOS-testing
then disable the base and whatever repo you have in your yum command. for eg to update php in c5-testing repo
yum --disablerepo=rpmforge,base update php