Extending Yii ============= Extending Yii is a common activity during development. For example, when you write a new controller, you extend Yii by inheriting its [CController] class; when you write a new widget, you are extending [CWidget] or an existing widget class. If the extended code is designed to be reused by third-party developers, we call it an *extension*. An extension usually serves for a single purpose. In Yii's terms, it can be classified as follows, * [application component](/doc/guide/basics.application#application-component) * [behavior](/doc/guide/basics.component#component-behavior) * [widget](/doc/guide/basics.view#widget) * [controller](/doc/guide/basics.controller) * [action](/doc/guide/basics.controller#action) * [filter](/doc/guide/basics.controller#filter) * [console command](/doc/guide/topics.console) * validator: a validator is a component class extending [CValidator]. * helper: a helper is a class with only static methods. It is like global functions using the class name as their namespace. * [module](/doc/guide/basics.module): a module is a self-contained software unit that consists of [models](/doc/guide/basics.model), [views](/doc/guide/basics.view), [controllers](/doc/guide/basics.controller) and other supporting components. In many aspects, a module resembles to an [application](/doc/guide/basics.application). The main difference is that a module is inside an application. For example, we could have a module that provides user management functionalities. An extension can also be a component that does not fall into any of the above categories. As a matter of fact, Yii is carefully designed such that nearly every piece of its code can be extended and customized to fit for individual needs.
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