yii2/docs/guide/structure-extensions.md

Extensions

Note: This section is under development.

Extensions are redistributable software packages that extend Yii by providing extra features. For example, the yii2-debug extension adds a handy debug toolbar to every page in your application to help you more easily grasp how the pages are generated. You can install and use extensions in your applications to accelerate your development process. You can also package your code in terms of extensions to share with other people your great work.

Getting Extensions

The easiest way of getting extensions is using Composer. The name "extension" is also known as "package" in Composer's terminology. To do so, you will need to

1. install Composer, which you probably have already done when installing Yii.
2. specify which repositories you would like to get extensions from. In most cases you can skip this if you only want to install open source extensions hosted on Packagist - the default and biggest Composer repository.
3. modify the composer.json file of your application and specify which extensions you want to use.
4. run php composer.phar install to install the specified extensions.

You usually only need to do Step 1 and 2 once. You may need to do Step 3 and 4 multiple times depending on your evolving requirements for extensions.

By default, extensions installed by Composer are stored under the BasePath/vendor directory, where BasePath refers to the application's base path.

For example, to get the yii2-imagine extension maintained officially by Yii, modify your composer.json like the following:

{
    // ...

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

        "yiisoft/yii2-imagine": "*"
    }
}

The installed extension will be located in the vendor/yiisoft/yii2-imagine directory.

Info: All extensions officially maintained by Yii are named as yiisoft/yii2-xyz, where xyz varies for different extensions.

Using Extensions

In order for your applications to use extensions, you will need to install Composer's class autoloader. This is necessary such that PHP classes in extensions can be properly autoloaded when you reference them in the application code. To do so, use the following lines in the entry script of your application:

// install Composer's class autoloader
require(__DIR__ . '/../vendor/autoload.php');

// include Yii class file
require(__DIR__ . '/../vendor/yiisoft/yii2/Yii.php');

Info: If you are using the basic application template or advanced application template, you do not need to do anything because the application templates already contain the above code.

Now you can enjoy using the installed extensions. For example, to use the yii2-imagine extension shown in the last subsection, you can write code like the following in your application, where Image is the class provided by the 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]);

Creating Extensions

Core Extensions