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Extensions are redistributable software packages specifically designed to be used in Yii applications and provide ready-to-use features. For example, the yiisoft/yii2-debug extension adds a handy debug toolbar at the bottom of every page in your application to help you more easily grasp how the pages are generated. You can use extensions to accelerate your development process. You can also package your code as extensions to share with other people your great work.

Info: We use the term "extension" to refer to Yii-specific software packages. For general purpose software packages that can be used without Yii, we will refer to them using the term "package" or "library".

Using Extensions

To use an extension, you need to install it first. Most extensions are distributed as Composer packages which can be installed by taking the following two simple steps:

  1. modify the composer.json file of your application and specify which extensions (Composer packages) you want to install.
  2. run composer install to install the specified extensions.

Note that you may need to install Composer if you do not have it.

By default, Composer installs packages registered on Packagist - the biggest repository for open source Composer packages. You can look for extensions on Packagist. You may also create your own repository and configure Composer to use it. This is useful if you are developing private extensions that you want to share within your projects only.

Extensions installed by Composer are stored in the BasePath/vendor directory, where BasePath refers to the application's base path. Because Composer is a dependency manager, when it installs a package, it will also install all its dependent packages.

For example, to install the yiisoft/yii2-imagine extension, modify your composer.json like the following:

    // ...

    "require": {
        // ... other dependencies

        "yiisoft/yii2-imagine": "*"

After the installation, you should see the directory yiisoft/yii2-imagine under BasePath/vendor. You should also see another directory imagine/imagine which contains the installed dependent package.

Info: The yiisoft/yii2-imagine is a core extension developed and maintained by the Yii developer team. All core extensions are hosted on Packagist and named like yiisoft/yii2-xyz, where xyz varies for different extensions.

Now you can use the installed extensions like they are part of your application. The following example shows how you can use the yii\imagine\Image class provided by the yiisoft/yii2-imagine extension:

use Yii;
use yii\imagine\Image;

// generate a thumbnail image
Image::thumbnail('@webroot/img/test-image.jpg', 120, 120)
    ->save(Yii::getAlias('@runtime/thumb-test-image.jpg'), ['quality' => 50]);

Info: Extension classes are autoloaded by the Yii class autoloader.

Installing Extensions Manually

In some rare occasions, you may want to install some or all extensions manually, rather than relying on Composer. To do so, you should:

  1. download the extension archive files and unpack them in the vendor directory.
  2. install the class autoloaders provided by the extensions, if any.
  3. download and install all dependent extensions as instructed.

If an extension does not have a class autoloader but follows the PSR-4 standard, you may use the class autoloader provided by Yii to autoload the extension classes. All you need to do is just to declare a root alias for the extension root directory. For example, assuming you have installed an extension in the directory vendor/mycompany/myext, and the extension classes are under the myext namespace, then you can include the following code in your application configuration:

    'aliases' => [
        '@myext' => '@vendor/mycompany/myext',

Creating Extensions

You may consider creating an extension when you feel the need to share with other people your great code. An extension can contain any code you like, such as a helper class, a widget, a module, etc.

It is recommended that you create an extension in terms of a Composer package so that it can be more easily installed and used by other users, as described in the last subsection.

Below are the basic steps you may follow to create an extension as a Composer package.

  1. Create a project for your extension and host it on a VCS repository, such as The development and maintenance work for the extension should be done on this repository.
  2. Under the root directory of the project, create a file named composer.json as required by Composer. Please refer to the next subsection for more details.
  3. Register your extension with a Composer repository, such as Packagist, so that other users can find and install your extension using Composer.


Each Composer package must have a composer.json file in its root directory. The file contains the metadata about the package. You may find complete specification about this file in the Composer Manual. The following example shows the composer.json file for the yiisoft/yii2-imagine extension:

    // package name
    "name": "yiisoft/yii2-imagine",

    // package type
    "type": "yii2-extension",

    "description": "The Imagine integration for the Yii framework",
    "keywords": ["yii2", "imagine", "image", "helper"],
    "license": "BSD-3-Clause",
    "support": {
        "issues": "",
        "forum": "",
        "wiki": "",
        "irc": "ircs://",
        "source": ""
    "authors": [
            "name": "Antonio Ramirez",
            "email": ""

    // package dependencies
    "require": {
        "yiisoft/yii2": "~2.0.0",
        "imagine/imagine": "v0.5.0"

    // class autoloading specs
    "autoload": {
        "psr-4": {
            "yii\\imagine\\": ""

Package Name

Each Composer package should have a package name which uniquely identifies the package among all others. The format of package names is vendorName/projectName. For example, in the package name yiisoft/yii2-imagine, the vendor name and the project name are yiisoft and yii2-imagine, respectively.

Do NOT use yiisoft as your vendor name as it is reserved for use by the Yii core code.

We recommend you prefix yii2- to the project name for packages representing Yii 2 extensions, for example, myname/yii2-mywidget. This will allow users to more easily tell whether a package is a Yii 2 extension.

Package Type

It is important that you specify the package type of your extension as yii2-extension so that the package can be recognized as a Yii extension when being installed.

When a user runs composer install to install an extension, the file vendor/yiisoft/extensions.php will be automatically updated to include the information about the new extension. From this file, Yii applications can know which extensions are installed (the information can be accessed via yii\base\Application::$extensions).


Your extension depends on Yii (of course). So you should list it (yiisoft/yii2) in the require entry in composer.json. If your extension also depends on other extensions or third-party libraries, you should list them as well. Make sure you also list appropriate version constraints (e.g. 1.*, @stable) for each dependent package. Use stable dependencies when your extension is released in a stable version.

Most JavaScript/CSS packages are managed using Bower and/or NPM, instead of Composer. Yii uses the Composer asset plugin to enable managing these kinds of packages through Composer. If your extension depends on a Bower package, you can simply list the dependency in composer.json like the following:

    // package dependencies
    "require": {
        "bower-asset/jquery": ">=1.11.*"

The above code states that the extension depends on the jquery Bower package. In general, you can use bower-asset/PackageName to refer to a Bower package in composer.json, and use npm-asset/PackageName to refer to a NPM package. When Composer installs a Bower or NPM package, by default the package content will be installed under the @vendor/bower/PackageName and @vendor/npm/Packages directories, respectively. These two directories can also be referred to using the shorter aliases @bower/PackageName and @npm/PackageName.

For more details about asset management, please refer to the Assets section.

Class Autoloading

In order for your classes to be autoloaded by the Yii class autoloader or the Composer class autoloader, you should specify the autoload entry in the composer.json file, like shown below:

    // ....

    "autoload": {
        "psr-4": {
            "yii\\imagine\\": ""

You may list one or multiple root namespaces and their corresponding file paths.

When the extension is installed in an application, Yii will create for each listed root namespace an alias that refers to the directory corresponding to the namespace. For example, the above autoload declaration will correspond to an alias named @yii/imagine.

Because extensions are meant to be used by other people, you often need to make an extra effort during development. Below we introduce some common and recommended practices in creating high quality extensions.


To avoid name collisions and make the classes in your extension autoloadable, you should use namespaces and name the classes in your extension by following the PSR-4 standard or PSR-0 standard.

Your class namespaces should start with vendorName\extensionName, where extensionName is similar to the project name in the package name except that it should not contain the yii2- prefix. For example, for the yiisoft/yii2-imagine extension, we use yii\imagine as the namespace for its classes.

Do not use yii, yii2 or yiisoft as your vendor name. These names are reserved for use by the Yii core code.

Bootstrapping Classes

Sometimes, you may want your extension to execute some code during the bootstrapping process stage of an application. For example, your extension may want to respond to the application's beginRequest event to adjust some environment settings. While you can instruct users of the extension to explicitly attach your event handler in the extension to the beginRequest event, a better way is to do this automatically.

To achieve this goal, you can create a so-called bootstrapping class by implementing yii\base\BootstrapInterface. For example,

namespace myname\mywidget;

use yii\base\BootstrapInterface;
use yii\base\Application;

class MyBootstrapClass implements BootstrapInterface
    public function bootstrap($app)
        $app->on(Application::EVENT_BEFORE_REQUEST, function () {
             // do something here

You then list this class in the composer.json file of your extension like follows,

    // ...

    "extra": {
        "bootstrap": "myname\\mywidget\\MyBootstrapClass"

When the extension is installed in an application, Yii will automatically instantiate the bootstrapping class and call its bootstrap() method during the bootstrapping process for every request.

Working with Databases

Your extension may need to access databases. Do not assume that the applications that use your extension will always use Yii::$db as the DB connection. Instead, you should declare a db property for the classes that require DB access. The property will allow users of your extension to customize which DB connection they would like your extension to use. As an example, you may refer to the yii\caching\DbCache class and see how it declares and uses the db property.

If your extension needs to create specific DB tables or make changes to DB schema, you should

  • provide migrations to manipulate DB schema, rather than using plain SQL files;
  • try to make the migrations applicable to different DBMS;
  • avoid using Active Record in the migrations.

Using Assets

If your extension is a widget or a module, chances are that it may require some assets to work. For example, a module may display some pages which contain images, JavaScript, and CSS. Because the files of an extension are all under the same directory which is not Web accessible when installed in an application, you have two choices to make the asset files directly accessible via Web:

  • ask users of the extension to manually copy the asset files to a specific Web-accessible folder;
  • declare an asset bundle and rely on the asset publishing mechanism to automatically copy the files listed in the asset bundle to a Web-accessible folder.

We recommend you use the second approach so that your extension can be more easily used by other people. Please refer to the Assets section for more details about how to work with assets in general.

Internationalization and Localization

Your extension may be used by applications supporting different languages! Therefore, if your extension displays content to end users, you should try to internationalize and localize it. In particular,

  • If the extension displays messages intended for end users, the messages should be wrapped into Yii::t() so that they can be translated. Messages meant for developers (such as internal exception messages) do not need to be translated.
  • If the extension displays numbers, dates, etc., they should be formatted using yii\i18n\Formatter with appropriate formatting rules.

For more details, please refer to the Internationalization section.


You want your extension to run flawlessly without bringing problems to other people. To reach this goal, you should test your extension before releasing it to public.

It is recommended that you create various test cases to cover your extension code rather than relying on manual tests. Each time before you release a new version of your extension, you may simply run these test cases to make sure everything is in good shape. Yii provides testing support, which can help you to more easily write unit tests, acceptance tests and functionality tests. For more details, please refer to the Testing section.


You should give each release of your extension a version number (e.g. 1.0.1). We recommend you follow the semantic versioning practice when determining what version numbers should be used.


To let other people know about your extension, you need to release it to the public.

If it is the first time you are releasing an extension, you should register it on a Composer repository, such as Packagist. After that, all you need to do is simply create a release tag (e.g. v1.0.1) on the VCS repository of your extension and notify the Composer repository about the new release. People will then be able to find the new release, and install or update the extension through the Composer repository.

In the releases of your extension, in addition to code files, you should also consider including the following to help other people learn about and use your extension:

  • A readme file in the package root directory: it describes what your extension does and how to install and use it. We recommend you write it in Markdown format and name the file as
  • A changelog file in the package root directory: it lists what changes are made in each release. The file may be written in Markdown format and named as
  • An upgrade file in the package root directory: it gives the instructions on how to upgrade from older releases of the extension. The file may be written in Markdown format and named as
  • Tutorials, demos, screenshots, etc.: these are needed if your extension provides many features that cannot be fully covered in the readme file.
  • API documentation: your code should be well documented to allow other people to more easily read and understand it. You may refer to the BaseObject class file to learn how to document your code.

Info: Your code comments can be written in Markdown format. The yiisoft/yii2-apidoc extension provides a tool for you to generate pretty API documentation based on your code comments.

Info: While not a requirement, we suggest your extension adhere to certain coding styles. You may refer to the core framework code style.

Core Extensions

Yii provides the following core extensions (or "Official Extensions") that are developed and maintained by the Yii developer team. They are all registered on Packagist and can be easily installed as described in the Using Extensions subsection.

The following official extensions are for Yii 2.1 and above. You don't need to install them for Yii 2.0, since they are included in the core framework.

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