We try to use the latest stable version of each product for improved security and bug fixes. In order to reduce breakage of scripts and such, we test each new version and only update to a new version if there is a significant improvement over the previous version.
Current Versions used:
PHP: We use CloudLinux which provides versions 4.4 thru 5.5+. You can change which PHP version your account uses in cPanel under "PHP Selector"