Step1:

Make a folder named "components" in your project root folder.

step2:

write your custom component inside components folder eg:MyComponent.php

namespace app\components;  use Yii;use yii\base\Component;use yii\base\InvalidConfigException; class MyComponent extends Component{ public function welcome() {  echo "Hello..Welcome to MyComponent"; } }

Step3:

Add your component inside the config/web.php file

'components' => [          'mycomponent' => [             'class' => 'app\components\MyComponent',                 ],           ]

Step4:

You are done!. Now you can use your component method "welcome" inside any of your app controller actions. eg:controllers/TestController.php

namespace app\controllers;use Yii; class TestController extends \yii\web\Controller{    public function actionWelcome()    {       Yii::$app->mycomponent->welcome();    } }