Yii is a fairly complex PHP framework that has a lot to offer. It is primarily meant for rapid development of large scale applications, it promotes code re-usability by component-based software engineering (CBSE). Going a step further in the separation of concerns direction, Yii is based on the Model View Controller (MVC) architectural pattern. The structure of a Yii application provides developers with a clearly designated hierarchy for each portion of the application thus motivating clear-cut code. The granularity of such an application structure while unusual can be highly effective.
In the same spirit, Yii has its own take on middleware, called filters in the framework, and database management among others. In short, it is a framework full of unique tools and approaches that can be very powerful in the right hands.