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The application object encapsulates the execution context within which a request is processed. Its main task is to collect some basic information about the request, and dispatch it to an appropriate controller for further processing. It also serves as the central place for keeping application-level configuration settings. For this reason, the application object is also called the front-controller.

The application object is instantiated as a singleton by the entry script. The application singleton can be accessed at any place via Yii::app().

1. Application Configuration

By default, the application object is an instance of CWebApplication. To customize it, we normally provide a configuration settings file (or array) to initialize its property values when it is being instantiated. An alternative way of customizing it is to extend CWebApplication.

The configuration is an array of key-value pairs. Each key represents the name of a property of the application instance, and each value the corresponding property's initial value. For example, the following configuration array sets the name and defaultController properties of the application.

    'name'=>'Yii Framework',

Note that application is a component like almost every Yii class. It means two things:

  • You can't set arbitrary properties as with plain PHP objects. A property must be defined in the application class.
  • Application supports setters to define a property so you can set, for example, a property defined by setImport method like the following:

We usually store the configuration in a separate PHP script (e.g. protected/config/main.php). Inside the script, we return the configuration array as follows:

return array(...);

To apply the configuration, we pass the configuration file name as a parameter to the application's constructor, or to Yii::createWebApplication() in the following manner, usually in the entry script:


Tip: If the application configuration is very complex, we can split it into several files, each returning a portion of the configuration array. Then, in the main configuration file, we can call PHP include() to include the rest of the configuration files and merge them into a complete configuration array.

2. Application Base Directory

The application base directory is the root directory under which all security-sensitive PHP scripts and data reside. By default, it is a subdirectory named protected that is located under the directory containing the entry script. It can be customized by setting the basePath property in the application configuration.

Contents under the application base directory should be protected against being accessed by Web users. With Apache HTTP server, this can be done easily by placing an .htaccess file under the base directory. The content of the .htaccess file would be as follows:

deny from all

3. Application Components

The functionality of the application object can easily be customized and enriched using its flexible component architecture. The object manages a set of application components, each implementing specific features. For example, it performs some initial processing of a user request with the help of the CUrlManager and CHttpRequest components.

By configuring the components property of the application instance, we can customize the class and property values of any application component used. For example, we can configure the CMemCache component so that it can use multiple memcache servers for caching, like this:

                array('host'=>'server1', 'port'=>11211, 'weight'=>60),
                array('host'=>'server2', 'port'=>11211, 'weight'=>40),

In the above, we added the cache element to the components array. The cache element states that the class of the component is CMemCache and its servers property should be initialized as such.

To access an application component, use Yii::app()->ComponentID, where ComponentID refers to the ID of the component (e.g. Yii::app()->cache).

An application component may be disabled by setting enabled to false in its configuration. Null is returned when we access a disabled component.

Tip: By default, application components are created on demand. This means an application component may not be created at all if it is not accessed during a user request. As a result, the overall performance may not be degraded even if an application is configured with many components. Some application components (e.g. CLogRouter) may need to be created regardless of whether they are accessed or not. To do so, list their IDs in the preload application property.

4. Core Application Components

Yii predefines a set of core application components to provide features common among Web applications. For example, the request component is used to collect information about a user request and provide information such as the requested URL and cookies. By configuring the properties of these core components, we can change the default behavior of nearly every aspect of Yii.

Here is a list the core components that are pre-declared by CWebApplication:

5. Application Life Cycle

When handling a user request, an application will undergo the following life cycle:

  1. Pre-initialize the application with CApplication::preinit();

  2. Set up the error handling;

  3. Register core application components;

  4. Load application configuration;

  5. Initialize the application with CApplication::init()

    • Register application behaviors;
    • Load static application components;
  6. Raise an onBeginRequest event;

  7. Process the user request:

    • Collect information about the request;
    • Create a controller;
    • Run the controller;
  8. Raise an onEndRequest event;

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