PHP Yii is a high-performance framework that combines core PHP with the solid design patterns of Java frameworks. The Yii framework includes three layers of abstractions. The first layer is the core engine that provides the foundation for web development. It handles all the low-level tasks such as routing and request/response handling. The second layer is the application layer which provides all sorts of components required by modern web applications like template engines, widgets, data providers etc. And finally, there is the third layer which contains extensions to Yii that add functionality to the core engine. It also provide additional tools to make it easier to develop web applications with Yii.
Yii is a free, open-source and developer-friendly framework. It provides classes for managing web applications in both PHP4 and PHP5. Within these classes are many features including MVC (model–view–controller), DAO (data access object), ActiveRecord, I18N/L10N support, caching, authentication and authorization, scaffolding and several other features.