This is my preferred way to upgrade PHP to 5.2 from the default 5.1.6 on CentOS 5.
You need to be root.
1) Make a reference to the CentOS5 Testing repo
Create the file /etc/yum.repos.d/centos-test.repo and put the following lines in it.
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[c5-testing] name=CentOS-5 Testing baseurl=http://dev.centos.org/centos/5/testing/$basearch/ enabled=0 gpgcheck=1 gpgkey=http://dev.centos.org/centos/RPM-GPG-KEY-CentOS-testing
2) Update php using yum as follows
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yum --enablerepo=c5-testing update php