after installing/removing various php installs from various repos - the final solution for php5.2 on centos5.3 is:

yum install yum-priorities

create a centos5test .repo file

vi /etc/yum.repos.d/c5-testing.repo

and paste in

[c5-testing]
name=CentOS-5 Testing
baseurl=http://dev.centos.org/centos/$releasever/testing/$basearch/
enabled=0
gpgcheck=1
gpgkey=http://dev.centos.org/centos/RPM-GPG-KEY-CentOS-testing

then run

yum –enablerepo=c5-testing install php

yum –enablerepo=c5-testing install php-devel php-pear

sudo pecl install PDO_MYSQL

sudo yum –enablerepo=c5-testing install php-mysql

sudo /etc/init.d/httpd restart