Oddly enough, PHP 5.2.6 is still not available on the default CentOS repositories ([base], [updates], [addons], and [extras]), the latest PHP version available in those repos is PHP 5.1.6!
In order to install PHP 5.2.6 you can add the CentOS testing repository and enable it then install or update PHP.
To add the CentOS testing directory, create a file /etc/yum.repos.d/centos-test.repo and add this line:
[c5-testing]
name=CentOS-5 Testing
baseurl=http://dev.centos.org/centos/5/testing/$basearch/
enabled=1
gpgcheck=1
gpgkey=http://dev.centos.org/centos/RPM-GPG-KEY-CentOS-testing
Then install or update PHP:
[root@server ~]# yum install php
or
[root@server ~]# yum update php
Then you can disable the testing repo by editing the file and setting the option “enabled” to “0”
[c5-testing]
name=CentOS-5 Testing
baseurl=http://dev.centos.org/centos/5/testing/$basearch/
enabled=0
gpgcheck=1
gpgkey=http://dev.centos.org/centos/RPM-GPG-KEY-CentOS-testing