Zend engine

Introduction

The Zend Engine has been the default PHP interpreter since PHP version 4. For PHP 5 a new engine was developed, known as Zend Engine II and for PHP 7 version III was developed also known as phpng.

The original engine was developed by the same two original authors that developed PHP.

What does the engine include?

The engine includes a compiler that can compile the PHP syntax. It will compile PHP from text files into something called "Zend opcodes". These opcodes can be executed by the runtime engine (which converts it into instructions that the operating system can execute and instructs the operating system to execute them).

https://github.com/php/php-src