Remove reliance on Zend API

• Version: 1.0
• Date: 2009-03-27
• Author: Paul Biggar paul.biggar@gmail.com
• Status: Early draft

A better way to provide a C API, with particular emphasis on decoupling extensions from the interpreter.

Naturally, this seems insane. Please bear with me.

The Zend API is a large set of functions, macros and data-structures which are used to interact with the Zend Engine. It serves 3 major purposes, roughly in order of importance:

• Used to write PHP's standard libraries, 3rd party extensions, and much of PECL
• Allows hot (performance-sensitive) code to be rewritten in C for speed
• Used to embed PHP into within C/C++ applications using the embed SAPI

The main problem with it is that it constrains the implementation of the Zend Engine. The Zend API creates a tight coupling between the ZendEngine and its users, restricting greatly our ability to change the Zend Engine. By requiring backwards compatability with the Zend Engine, we are ensuring that the ZendEngine can only be modified in minor ways (although ABI changes are allowed for major versions). This holds the Zend Engine to design decisions made nearly 10 years ago, and prevent PHP from getting much faster in the long term.

The Zend API also makes it difficult to write PHP extensions. Although most of the API is not terribly difficult to work with, concepts like copy-on-write, change-on-write sets and separation appear to be tricky concepts for many people. The only documentation is Sara Golemon's book, and the actual code is not well commented. I believe that a simpler way of writing extensions would be welcome.

A number of other PHP implementations exist, such as IBM's Project Zero, Phalanger, Roadsend, Quercus and phc. Many of these projects find it very difficult to re-use PHP's standard libraries. Quercus and Roadsend have reimplemented popular standard libraries. Phalanger and Project Zero attempt to re-use the existing libraries by marshalling their data into the Zend API. This appears to be slow and error-prone. phc is designed around reusing the Zend API for compatibility with the PHP. This constrains many of the optimizations phc would wish to perform, since it uses the Zend API nearly everywhere.