The Zend frame work is an open source oop framework that is run on PHP 5. It is licensed under the commercially friendly New BSD License to make it appropriate for buisienss use. It was started in 2005 to compete with existing web frameworks such as Ruby on Rails and has quickly grown, with Zend.com now reporting over 10 million downloads. The support has also quickly grown to what you would expect from an enterprise level framework, with a 1000+ page reference guide and over 500 examples. The Zend framework has a learning curve that is simliarly quick to that of PHP, developers can fall back into standard php when required and standard design patterns such as MVC are supported but not required.
The Zend Server is also available to allow you to run the Zend Framework with native OS support and caching optimisations; though the Zend framework can run on other servers such as Apache, the Zend server provides optimisations such as opcode support for quick execution at the native level. Zend studio provides easy generation of MVC code generation and tightly integrated editing for rapid development of Zend framework applications. Zend Studio costs, in contrast to the Zend Framework itself and the community edition of Zend Server which are both free. There’s a really great rundown of the zend framework on snook.ca if you fancy having a look.
All code that is merged into the main branch of the Zend Framework requires unit tests that cover 80% of the code meaning any stable branches are very stable.