The Base tier extends the Core tier to provide more general but still often used functionality, as well as sane defaults. This extended functionality helps the developer to focus on building a web application, and not an application framework.
It forms the basis on which the Backend-PHP Framework is built.
It can be installed from source.
Base uses Backend-PHP Interfaces and Backend-PHP Core, and as this packages uses namespaces, PHP 5.3 is required.