PHP is such a huge and complex package that its inner workings can only be understood if you make yourself familiar with it by practicing, so we encourage you to work with the source.
The name Zend refers to the language engine, PHP's core. The term PHP refers to the complete system as it appears from the outside. This might sound a bit confusing at first, but it's not that complicated (see 그림 24-1). To implement a Web script interpreter, you need three parts: