Joomla! 1.6 requires PHP 5.3 or later to be installed. When requesting a new server, this is far less of a problem as many will be provisioned with PHP 5.3 already installed. What should you do, however, if you are running a version of CentOS prior to 5.6 you will need to manually update PHP before Joomla 1.6 will install.
In fact, even with Joomla 1.5 you may experience issues if you are running an earlier version of PHP. The latest build of Akeeba Backup, for example, requires PHP 5.3.
This post will detail the method you need to use to update from PHP 5.1 or 5.2 to PHP 5.3 (Note: These instructions should not be followed to update from PHP4 unless you know exactly what you are doing!)
