Yii is a high-performance component-based PHP framework best for developing large-scale Web applications. Yii comes with a full stack of features, including MVC, DAO/ActiveRecord, I18N/L10N, caching, jQuery-based AJAX support, authentication and role-based access control, scaffolding, input validation, widgets, events, theming, Web services, and so on. Written in strict OOP, Yii is easy to use and is extremely flexible and extensible.

Features:

• MVC design pattern
• DAO and Active Record
• jQuery-based JavaScript support
• I18N and L10N
• Page, fragment and data caching
• Error handling and logging
• Theming
• Web services
• Console applications
• Authentication and authorization
• Web 2.0 widgets
• Form input and validation

Performance comparison:

• Yii: 427 requests/second
• CakePHP: 170 requests/second
• CodeIgniter: 131 requests/second
• Prado: 53 requests/second
• Symfony: 51 requests/second
• Zend: 36 requests/second

Credits:
Yii incorporates many ideas and work from other well-known Web programming frameworks and applications. Below is a short list.

• Prado: This is the major source of ideas for Yii. Yii adopts its component-based and event-driven programming paradigm, database abstraction layers, modular application architecture, internationalization and localization, and many others.
• Ruby on Rails: Yii inherits its spirit of conventions over configurations. Yii also referenced its implementation of active record design pattern.
• jQuery: This is integrated in Yii as the foundational JavaScript framework.
• Symfony: Yii referenced its filter design and plug-in architecture.
• Joomla: Yii referenced its modular design and message translation scheme.

Documentation:
The Guide - The Definitive Guide to Yii is the most comprehensive documentation about Yii. It gives definitive description of Yii's features and is a good start for beginners.
[download as PDF]

Class Reference - Yii Framework Class Reference gives every detail about properties, methods and events defined in Yii framework classes.
[download as CHM]

License:
Yii is released under the BSD License. This means you can use it for free to develop either open source or proprietary Web applications.