Zend Framework requires PHP 5.2.4 and up. Some Zend Framework components may work with earlier versions of PHP, but these components are tested and supported only on 5.2.4 and up. See the requirements appendix for more information.
Does Zend Framework support PHP 4?
No. Zend Framework was built to use all of the sophisticated object oriented features of PHP 5 and take advantage of significant performance and security enhancements.
Another consideration was support of the platform ZF would be running on. The PHP community officially discontinued support for PHP 4 as of 2008-01-01, and no critical security updates will be published for PHP 4 after 2008-08-08.
These factors, among others, convinced us that PHP 5 was the best platform for Zend Framework and applications built on ZF.
Where is the model in ZF's MVC implementation?
The model component can vary dramatically in responsibilities and data store from one MVC application to the next. The ZF community has not defined a model interface, class, or other formalism simply because we wanted to avoid introducing limitations without significant added value.
Is ZF a component library or a framework?
Simple answer: both. Zend Framework provides all the components required for most web applications in a single distribution. But Zend Framework components are also loosely coupled, making it easy to use just a few components in a web application- even alongside other frameworks! Using this use-at-will architecture, we are implementing features commonly found in more monolithic frameworks. In fact, we are currently working on a tooling component for the 1.8 release that will make it simpler to build applications using ZF components, yet will not sacrifice the use-at-will nature of existing ZF components. It's a testament to the use-at-will architecture of Zend Framework that the tooling component itself can be used standalone.