The Zend Engine provides an extension API. This allows anyone to write classes, functions, constants and much more that plug in to the php.net runtime. Taking a peek at the php.net source code in CVS reveals just how many extensions there are.

[...] Zero has a very similar architecture that separates engine and extension. The main difference is that the core PHP language engine is written in Java. Zero also has a comprehensive API called XAPI-J that allows Java extensions to be written for the Zero PHP runtime.

The main difference between the two is the obvious recompile that the Project Zero extensions will have to go through (via JNI) to work correctly with their version of the core. This allows it to be able to use just about any PHP extension out there and saves a lot of work on recoding the great functionality already there.