Zend Engine 2 API reference
Old introduction
PHP at the Core: A Hacker's Guide to the Zend Engine
PHP Manual

# Zend Engine 1

## Table of Contents

- Old introduction
- Streams API for PHP Extension Authors
- Zend API: Hacking the Core of PHP
- TSRM API

Zend Engine 1 is the internal engine used by PHP for the entire version 4 release line. It is no longer considered active, but PHP 4 is still in widespread use, so the old ZE1 documentation is preserved here exactly as it was.

Zend Engine 2 API reference
Old introduction
PHP at the Core: A Hacker's Guide to the Zend Engine
PHP Manual