The IBM developerWorks site has posted a very high-level overview of one of the most popular PHP frameworks currently in use the Zend Framework. This is part one of their introductory series looking at the framework and what it has to offer.
Technical alliances with companies such as Abobe, Microsoft, and IBM, have strengthened the support and stability of the framework, which is now used by startups and Fortune 500 companies around the world. This article gives you a high-level view of the Zend Framework, explaining its general concepts and preparing you for the rest of this "Understanding the Zend Framework" series, which goes into the details by chronicling the creation of a new online RSS/Atom feed reader. You won't do much coding here, but for the rest of the series, you should be familiar with PHP.
They introduce the framework and talk about some if its origins and how the Zend Framework found its niche to stand on and dramatically improve over the past few years. They list some of the more widely used components including the Zend_Form, Zend_Db, Zend_Filter, Zend_Search, Zend_View and Zend_Log.
This series was originally published about four years ago but they've gone through and given it a refresh. The result is this first part and eight others with the holes filled in for the latest version and features of the framework. This first part has links to the others in the series near the bottom.