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Aliases help you not to hard-code absolute paths or URLs in your project. An alias starts with the @ character.

To define an alias you should call the Yii::setAlias() method −

// an alias of a file path
Yii::setAlias('@alias', '/path/to/alias');
// an alias of a URL
Yii::setAlias('@urlAlias', 'http://www.google.com');

You can also derive a new alias from an existing one −

Yii::setAlias('@pathToSomewhere', '@alias/path/to/somewhere');

You can call the Yii::setAlias() method in the entry script or in a writable property called aliases in the application configuration −

$config = [
   'id' => 'basic',
   'basePath' => dirname(__DIR__),
   'bootstrap' => ['log'],
   'components' => [
      'aliases' => [
         '@alias' => '/path/to/somewhere',
         '@urlAlias' => 'http://www.google.com',
      ],
      //other components...
   ]
]

To resolve alias, you should call the Yii::getAlias() method.

Yii predefines the following aliases −

• @app − The base path of the application.
• @yii − The folder where the BaseYii.php file is located.
• @webroot − The Web root directory of the application.
• @web − The base URL of the application.
• @runtime − The runtime path of the application. Defaults to @app/runtime.
• @vendor − The Composer vendor directory. Defaults to @app/vendor.
• @npm − The root directory for npm packages. Defaults to @vendor/npm.
• @bower − The root directory for bower packages. Defaults to @vendor/bower.

Now, add a new function called actionAliases() to the SiteController −

public function actionAliases() {
   Yii::setAlias("@components", "@app/components");
   Yii::setAlias("@imagesUrl", "@web/images");
   var_dump(Yii::getAlias("@components"));
   var_dump(Yii::getAlias("@imagesUrl"));
}

In the above code, we created two aliases: @components for application components and @imagesUrl for URL where we stored all application images.

Type http://localhost:8080/index.php?r=site/aliases, you will see the following output −