The Zend Engine
When PHP 3 was written, an entirely new parser was created from the ground up. The introduction of PHP 4 represented a similar revolution in the code base. Zend is a scripting engine that sits below the PHP-specific modules. It was optimized to ensure massively improved performance and extensibility.
PHP 5 brings new fundamental improvements with the introduction of the Zend Engine 2. We have already touched on the great change ushered in by ZE2. The engine provides significantly enhanced support for object-oriented programming. For the first time, objects and object-oriented design lie at the heart of PHP, making it an even more suitable platform for large enterprise applications.