Zend Engine is an open source scripting engine just like engine in a car. The Zend Engine is developed by two students Zeev Suraski and Andi Gutmans (at the Technion – Israel Institute of Technology) in 1999 for PHP4 called Zend Engine-1, and the current is for PHP5 called as Zend Engine-2. Before these engines we were using old and less powerful engine developed by the inventor of PHP Rasmus Lerdorf.
Here is the brief of zend engine, it consists of fallowing:
- Interpreter(Run Time Compiler)
- Executor
- Function Module Interface
I am expecting that we all know what is interpreter so there is no need to discuss it. The latest feature is Extension Interface that makes it extensible, like Zend Debugger,Zend Optimizer,Zend Persistent compiled Script Registry and Zend script to binary file compiler, this feature is not depicted in diagram. Executor is responsible for finally executing the code and generating output. Function module interface is used for including different functionality like MySql,Oracle,PostgreeSQL,XML,Math,Com Components, ODBC, WDDX etc. So the conclusion is Zend Engine is just a virtual machine that is responsible for including different modules to finally providing the output, the interpreter is just a small part of it.
Authors bio is coming up shortly. 
do you know if or how the zend engine interpreter can be extended?
id love to extend it with an alternative syntax for php.
and handle it somehow like this
my idea:
MOFL => MyOwnFunnyLanguage