Path Alias and Namespace

Yii uses path aliases extensively. A path alias is associated with a directory or file path. It is specified in dot syntax, similar to that of widely adopted namespace format:

where RootAlias is the alias of some existing directory.

By using YiiBase::getPathOfAlias(), an alias can be translated to its corresponding path. For example, system.web.CController would be translated as yii/framework/web/CController.

We can also use YiiBase::setPathOfAlias() to define new root path aliases.

1. Root Alias

For convenience, Yii predefines the following root aliases:

  • system: refers to the Yii framework directory;
  • zii: refers to the Zii library directory;
  • application: refers to the application's base directory;
  • webroot: refers to the directory containing the entry script file.
  • ext: refers to the directory containing all third-party extensions.

Additionally, if an application uses modules, each module will have a predefined root alias that has the same name as the module ID and refers to the module's base path. For example, if an application uses a module whose ID is users, a root alias named users will be predefined.

2. Importing Classes

Using aliases, it is very convenient to include the definition of a class. For example, if we want to include the CController class, we can call the following:


The import method differs from include and require in that it is more efficient. The class definition being imported is actually not included until it is referenced for the first time (implemented via PHP autoloading mechanism). Importing the same namespace multiple times is also much faster than include_once and require_once. Note that importing a directory does not import any of its subdirectories.

Tip: When referring to a class defined by the Yii framework, we do not need to import or include it. All core Yii classes are pre-imported.

Using Class Map

Starting from version 1.1.5, Yii allows user classes to be pre-imported via a class mapping mechanism that is also used by core Yii classes. Pre-imported classes can be used anywhere in a Yii application without being explicitly imported or included. This feature is most useful for a framework or library that is built on top of Yii.

To pre-import a set of classes, the following code must be executed before CWebApplication::run() is invoked:

    'ClassName1' => 'path/to/ClassName1.php',
    'ClassName2' => 'path/to/ClassName2.php',

3. Importing Directories

We can also use the following syntax to import a whole directory so that the class files under the directory can be automatically included when needed.


Besides import, aliases are also used in many other places to refer to classes. For example, an alias can be passed to Yii::createComponent() to create an instance of the corresponding class, even if the class file was not included previously.

4. Namespace

A namespace refers to a logical grouping of some class names so that they can be differentiated from other class names even if their names are the same. Do not confuse path alias with namespace. A path alias is merely a convenient way of naming a file or directory. It has nothing to do with a namespace.

Tip: Because PHP prior to 5.3.0 does not support namespace intrinsically, you cannot create instances of two classes who have the same name but with different definitions. For this reason, all Yii framework classes are prefixed with a letter 'C' (meaning 'class') so that they can be differentiated from user-defined classes. It is recommended that the prefix 'C' be reserved for Yii framework use only, and user-defined classes be prefixed with other letters.

5. Namespaced Classes

A namespaced class refers to a class declared within a non-global namespace. For example, the application\components\GoogleMap class is declared within the namespace application\components. Using namespaced classes requires PHP 5.3.0 or above.

Starting from version 1.1.5, it is possible to use a namespaced class without including it explicitly. For example, we can create a new instance of application\components\GoogleMap without including the corresponding class file explicitly. This is made possible with the enhanced Yii class autoloading mechanism.

In order to be able to autoload a namespaced class, the namespace must be named in a way similar to naming a path alias. For example, the class application\components\GoogleMap must be stored in a file that can be aliased as application.components.GoogleMap.

So to use custom namespace starting with, for example, \mynamespace where classes are located at /var/www/common/mynamespace/, the only thing you should do is to define a path alias like the following:

Yii::setPathOfAlias('mynamespace', '/var/www/common/mynamespace/');

6. Namespaced Controllers

By default Yii uses controllers from the global namespace. These classes are located under protected/controllers. You can change this behavior in two different ways: using controllerMap and using controllerNamespace. The former allows you to use controllers from various namespaces. The latter requires less configuration while setting a common namespace for all controllers.

Using controllerMap

The best way to change controller map is to use the configuration file (protected/config/main.php):

// adding "mynamespace" namespace
Yii::setPathOfAlias('mynamespace', '/var/www/common/mynamespace/');
return array(
    'name'=>'My Web Application',
    'controllerMap' => array(
        'test' => '\mynamespace\controllers\TestController',

When user tries to load any of the controllers defined in controllerMap, Yii loads the specified classes, bypassing the normal controller loading method. In case of test Yii will load the namespaced class \mynamespace\controllers\TestController located at /var/www/common/mynamespace/controllers/TestController.php.

Note that controller code should be properly namespaced:

// define namespace:
namespace mynamespace\controllers;
// since class is now under namespace, global namespace
// should be referenced explicitly using "\":
class TestController extends \CController
    public function actionIndex()
        echo 'This is TestController from \mynamespace\controllers';

Using controllerNamespace

Since application is the module itself, you can use controllerNamespace property in the same way as described in "Namespaced Modules" below.

7. Namespaced Modules

Sometimes it's useful to namespace the whole module. For example, if you want to put testmodule under \mynamespace\modules\testmodule pointing to /var/www/common/mynamespace/modules/testmodule you should first create the following file structure:


index.php view is the same as in regular module. TestmoduleModule.php and DefaultController.php are namespaced.


// define namespace:
namespace mynamespace\modules\testmodule;
// since class is now under namespace, global namespace
// should be referenced explicitly using "\":
class TestmoduleModule extends \CWebModule
    // setting non-global controllers namespace (also can be done via config)
    public $controllerNamespace = '\mynamespace\modules\testmodule\controllers';
    // usual module code


// define namespace:
namespace mynamespace\modules\testmodule\controllers;
// since class is now under namespace, global namespace
// should be referenced explicitly using "\":
class DefaultController extends \Controller
    public function actionIndex()

Now the only thing left is to add our module to the application. The best way to do it is to specify it in the application config file (protected/config/main.php):

// adding "mynamespace" namespace
Yii::setPathOfAlias('mynamespace', '/var/www/common/mynamespace/');
return array(
    'name'=>'My Web Application',
        'testmodule' => array(
            'class' => '\mynamespace\modules\testmodule\TestModuleModule',

Total 3 comments

#18813 report it
marcovtwout at 2015/01/14 07:06am
Module alias not always defined

Normally, a module alias is immediately available in your application. But if you declare your module like this:

'modules' => array(
    'bar' => array(
        'class' => ''

The bar alias is not available until the module has been instantiated (Yii::app()->getModule('bar')). This limitation is not likely to be fixed.

#10572 report it
Webnet668 at 2012/11/05 12:17pm
Using Yii::app() with namespaces

When using namespaces, you might see an error about the application not being able to find "Yii.php". If you run into this, there are 2 solutions.

1) Instead of

namespace application\components\foo;
class Controller extends \CController {
    public function actionIndex() {
         $request = Yii::app()->getRequest(); //Error: could not find Yii.php

Tell PHP you want it to look within the Yii namespace so it knows where to find any Yii objects...

namespace application\components\foo;
use Yii;
class Controller extends \CController {
    public function actionIndex() {
         $request = Yii::app()->getRequest(); //doesn't throw an error

2) Change all instances of




Though ideally you would use the first solution.

#3341 report it
stefanos at 2011/04/05 12:34am
Namespaces in a nutshell

Namespaces in php are like directory paths in console (bash, dos etc)
When you use namespace php keyword like this

namespace a\random\namespace;
  class Foo {
    static function bar(){} 

is like executing cd a\specific\directory except that the namespace is created if not exists.
Now everything follows is belonging to that namespace. This means that if you want to instantiate, extend or call a static method from eg foo class on another namespace you have to

//The leading backslash '\' here denotes the global namespace that
//is the root folder or C:\ counterpart from console environment 
$baz= new \a\random\namespace\Foo; 
class Fighter extends \a\random\namespace\Foo {}

Yii imports a very intuitive convention here that the namespace structure (if implemented) should be reflected on the physical directory structure and additionally makes its Path Alias convenience available for that purpose.
Please be my guest to follow these steps:
1. Create a new web app 2. Go to protected\components and create a folder foo 3. Move Controller.php in foo folder and open it with an editor 4. At line 6, at Controller class declaration import this:

namespace application\components\foo;
class Controller extends \CController //Note the leading backslash here
  1. Now open protected\controllers\SiteController.php for editing
  2. Replace the SiteController class declaration with this
class SiteController extends \application\components\foo\Controller

As you will see, your new web app still working fine and application path alias will point properly at protected folder.
You can find more about php namespaces here
Enjoy coding :>

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