For this we will use the CentOS testing repository:

cd /etc/yum.repos.d/
sudo wget http://dev.centos.org/centos/5/CentOS-Testing.repo
sudo rpm --import http://dev.centos.org/centos/RPM-GPG-KEY-CentOS-testing


Now we can fetch package information about PHP using the new repository:

yum --enablerepo=c5-testing info php


Check the version number. If it is what you need you can go ahead and install it using yum:

sudo  yum --enablerepo=c5-testing update php


Assuming that worked, you can now restart Apache and be on your way:

sudo /etc/init.d/httpd restart


Memcache

If, like me, you were using memcache you might run into a problem where memcache never got updated to the proper version because it did not exist in the CentOS testing repository. The memcache module on the system ends up with a different PHP API version than is required by the new version of PHP you just installed.

Upon starting PHP you might see a warning similar to this:

Module compiled with API 20040429
PHP compiled with API 20060613


Again, we will add a new repository from Remi Collet. The steps should be as follows:

cd /etc/yum.repos.d/
wget http://rpms.famillecollet.com/remi-enterprise.repo
wget http://download.fedora.redhat.com/pub/epel/5/i386/epel-release-5-3.noarch.rpm
wget http://rpms.famillecollet.com/enterprise/remi-release-5.rpm
rpm -Uvh remi-release-5*.rpm epel-release-5*.rpm


Now we can install the newest memcache library:

sudo yum --enablerepo=remi install libmemcached\*


Build the PHP module using Pecl (you might have to install php-devel packages for this):

pecl install memcache


Edit /etc/php.d/memcache.ini and add:

extension=memcache.so


Restart Apache:

sudo /etc/init.d/httpd restart


With a little bit of luck you will now have PHP 5.2.x and Memcache!