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Aliases are used to represent file paths or URLs so that you don't have to hard-code absolute paths or URLs in your project. An alias must start with the @ character to be differentiated from normal file paths and URLs. Alias defined without leading @ will be prefixed with @ character.

Default Yii application has some aliases pre-defined in config/params.php. For example, the alias @src represents the installation path of application src directory; @web represents the base URL for the currently running Web application.

Defining Aliases

You can define an alias via application's config/params.php:

return [
    // ...

    'aliases' => [
        // an alias of a file path
        '@foo' => '/path/to/foo',
        // an alias of a URL
        '@bar' => '',
        // an alias of a concrete file that contains a \foo\Bar class 
        '@foo/Bar.php' => '/definitely/not/foo/Bar.php',

Note: The file path or URL being aliased may not necessarily refer to an existing file or resource.

Given a defined alias, you may derive a new alias by appending a slash / followed with one or more path segments. For example, @foo is a root alias, while @foo/bar/file.php is a derived alias.

You can define an alias using another alias (either root or derived):

'@foobar' => '@foo/bar', 

The aliases parameter initializes Aliases service from yiisoft/aliases package. You can set additional aliases in runtime by using the service:

use \Yiisoft\Aliases\Aliases;

public function actionIndex(Aliases $aliases)
    $aliases->set('@uploads', '@root/uploads');

Resolving Aliases

You can use Aliases service to resolve an alias or derived alias into the file path or URL it represents:

use \Yiisoft\Aliases\Aliases;

public function actionIndex(Aliases $aliases)
    $foo = $aliases->get('@foo'); // /path/to/foo
    $bar = $aliases->get('@bar'); //
    $file = $aliases->get('@foo/bar/file.php'); // /path/to/foo/bar/file.php

The path/URL represented by a derived alias is determined by replacing the root alias part with its corresponding path/URL in the derived alias.

Note: The get() method does not check whether the resulting path/URL refers to an existing file or resource.

A root alias may also contain slash / characters. The get() method is intelligent enough to tell, which part of an alias is a root alias and thus correctly determines the corresponding file path or URL:

use \Yiisoft\Aliases\Aliases;

public function actionIndex(Aliases $aliases)
    $aliases->set('@foo', '/path/to/foo');
    $aliases->set('@foo/bar', '/path2/bar');

    $aliases->get('@foo/test/file.php'); // /path/to/foo/test/file.php
    $aliases->get('@foo/bar/file.php'); // /path2/bar/file.php

If @foo/bar is not defined as a root alias, the last statement would display /path/to/foo/bar/file.php.

Predefined Aliases

Yii predefines a set of aliases to easily reference commonly used file paths and URLs:

  • @root, the base directory of the currently running application.
  • @vendor, Composer's vendor directory.
  • @public, the web root directory of the currently running Web application.
  • @runtime, the runtime path of the currently running application. Defaults to @root/runtime.
  • @web, the base URL of the currently running Web application.

Package Aliases

An alias is automatically defined for each extension that is installed via Composer. Each alias is named after the root namespace of the extension as declared in its composer.json file, and each alias represents the root directory of the package. For example, if you install the yiisoft/yii2-jui extension, you will automatically have the alias @yii/jui defined during the bootstrapping stage, equivalent to:

Yii::setAlias('@yii/jui', 'VendorPath/yiisoft/yii2-jui');
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