By default Plesk installation, the PHP version is 5.1.x, if you want to upgrade it to 5.2 or even 5.3, please follow these steps:
Login via SSH as root user first.
Upgrading PHP:
wget -q -O - http://www.atomicorp.com/installers/atomic.sh | sh
yum update php
yum update php
And then follow on-screen steps.
