Panda PHP is a powerful set of useful PHP components to make everyday PHP development easier.
Although Panda PHP isn't a framework itself, a simple MVC-style framework has be written using Panda PHP, and is included with the source distribution.
Everything in Panda PHP is an object. In order to use Panda PHP to its fullest, you should be familiar with object-oriented concepts such as inheritance, abstraction, interfaces, and typehinting.
Panda PHP makes heavy use of PHP 5 interceptors and SPL therefore the is no hope for a backport to PHP 4. This is a good thing.