Unfortunately RHEL 5 does not have php.5.2 package, which is required by most of the applications including latest wordpress and drupal.
First thought of compiling php from source, but hard to keep it uptodate. So decided to make the life easier with EPEL/IUS repositories.
Remove all existing php related packages:
# rpm -e php php-mysql php-cli php-pdo php-common
Download and install the EPEL/IUS RPMs
# wget http://dl.iuscommunity.org/pub/ius/stable/Redhat/5/i386/epel-release-1-1.ius.el5.noarch.rpm
# wget http://dl.iuscommunity.org/pub/ius/stable/Redhat/5/i386/ius-release-1.0-8.ius.el5.noarch.rpm
incase if the list not working just browse and find the rpm.
Install the RPMs
# rpm -Uvh *-release-*.rpm
Now you can install php 5.2 or 5.3 like:
# yum install php52 php52-mysql