<p>I am running a site about PHP Frameworks, but I can't find a exact definition for it, and I am always thinking a question, how to make out a good PHP framework? Features? Manual? Efficiency? Or something?</p>

A PHP Framework in my eyes is a collection of classes which help you develop a web application.
In my company we use the Zend Framework. I have to say that getting started with this Framework is pretty hard, but if you get how to use the API and the Reference Guide you have a great Framework. Also its interessting because they have Zend people sitting on the project so they can use those internal tricks and get some performance boosts others can't get.
What also differs ZF from others is the ability to create MVC programms with no big hassles.