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Yii is a high-performance PHP framework best for developing Web 2.0 applications. Yii comes with rich features: MVC, DAO/ActiveRecord, I18N/L10N, caching, authentication and role-based access control, scaffolding, testing, etc. It can reduce your development time significantly.

FAST
Yii only loads the features that you need. It has powerful caching support. It is explicitly designed to work efficiently with AJAX.

SECURE
Security comes as standard with Yii. It includes input validation, output filtering, SQL injection and Cross-site scripting prevention.

PROFESSIONAL
Yii helps you develop clean and reusable code. It follows the MVC pattern, ensuring a clear separation of logic and presentation.

FEATURES :
- Model-View-Controller (MVC) design pattern

Yii adopts the proven MVC architecture, which allows for clean separation of concerns (SoC).
-Database Access Objects (DAO), Query Builder, Active Record, DB Migration

Yii allows developers to model database data in terms of objects and avoid the tedium and complexity of writing repetitive SQL statements.
- Form input and validation

Yii makes collecting form input extremely easy and safe. It comes with a set of validators as well as numerous helper methods and widgets to simplify the task for form input and validation.
- AJAX-enabled widgets

Integrated with jQuery, Yii comes with a set of AJAX-enabled widgets, such as auto-complete input field, treeview, data grid, which enables writing highly efficient and versatile user interface extremely simple.
- Authentication and authorization

Yii has built-in authentication support. It also supports authorization via hierarchical role-based access control (RBAC).
- Skinning and theming

Yii implements a skinning and theming mechanism that allows you to quickly switch the outlook of a Yii-power website.
- Web services

Yii supports automatic generation of complex WSDL service specifications and management of Web service request handling. Internationalization (I18N) and localization (L10N) Yii supports message translation, date and time formatting, number formatting, and interface localization.
- Layered caching scheme

Yii supports data caching, page caching, fragment caching and dynamic content. The storage medium of caching can be changed easily without touching the application code.
- Error handling and logging

Errors are handled and presented more nicely, and log messages can be categorized, filtered and routed to different destinations.
- Security

Yii is equipped with many security measures to help prevent your Web applications from attacks such as SQL injection, cross-site scripting (XSS), cross-site request forgery (CSRF), and cookie tampering.
- Unit and functionality testing

Yii provides support for writing and running unit tests as well as functionality tests, based on PHPUnit and Selenium.
- Automatic code generation

Yii provides a set of intuitive and highly extensible code generation tools that can help you quickly generate the code you need for features such as form input, CRUD.
- Compliance to XHTML

HTML code produced by Yii components and code generators complies to the XHTML standard.
- Purely object-oriented

Yii framework sticks to strict OOP paradigm. It does not define any global function or variable. And the class hierarchy that it defines allows maximum reusability and customization.
- Friendly with third-party code

Yii is carefully designed to work well with third-party code. For example, you can use code from PEAR or Zend Framework in your Yii application.
- Detailed documentation

Every single method or property is clearly documented. A book and comprehensive tutorials are also available for you to systematically learn Yii.
- Extension library

Yii provides an extension library consisting of user-contributed components. This makes the above feature list never ending.

ref/source :http://www.yiiframework.com/

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