Like its little brother, the PHP3 engine, Zend comes to solve several deficiencies in previous versions of PHP. First, and perhaps most importantly, it implements the same language as PHP 3.0, but does so in a much more optimized manner. Zend typically executes PHP3 scripts at anywhere between 3 to 200 times quicker. Considering you don’t have to change a single letter in your script, but simply install a new version of PHP – that’s a pretty important benefit. This benefit is by far most noticeable on extremely busy sites, or sites that implement complex logic in their PHP scripts. Zend plugins, that will be available separately, will allow even higher speed for sites that need to squeeze every last bit of performance.

Zend introduces several new language constructs (like foreach, class::method() calls, reference support and more), and also remedies the most annoying problems of the PHP3 engine (notably, nesting arrays within objects and vice versa; a true implementation of unset(); and more). For a full list of new language-level features or improvements, go to http://www.zend.com/, and select the New Features button.

In order to better understand how PHP relates to its engine (which is very similar to the relationship between a car and its engine), consider the following diagram:

As you can see, without the PHP modules and the wrapper layer, all you would have is a nifty scripting engine, but not something you can actually work with.