The minimum required version for PartKeepr is 5.6.0. Several distributions don't have PHP 5.6 yet. Luckily, there are packages for most distributions.
RHEL and CentOS
You can install PHP 5.6 for RedHat/CentOS using Webtatic.
Ubuntu 14.04 LTS and 14.10
Please use the PPA for PHP 5.6.
Debian Wheezy and Squeeze
Use the dotdeb packages.