how to Json encode in yii2?

Solution 1: In case if all your controller's actions will deliver json you may also consider extanding yii\rest\Controller instead of yii\web\Controller :

namespace app\controllers;

use Yii;

class UserController extends \yii\rest\Controller
{
    public function actionRegister()
    {
        $username = Yii::$app->request->post('username');
        return $username;
    }
}

NOTE: you may also use ActiveController which extends yii\rest\Controller (see rest docs) if you need to handle CRUD operations.

Solution 2: A different approach when extending yii\web\Controller is by using yii\filters\ContentNegotiator. Note that setting $enableCsrfValidation to false may be mandatory here as it is explained in its related docs :

Whether to enable CSRF (Cross-Site Request Forgery) validation. Defaults to true. When CSRF validation is enabled, forms submitted to an Yii Web application must be originated from the same application. If not, a 400 HTTP exception will be raised.

Note, this feature requires that the user client accepts cookie. Also, to use this feature, forms submitted via POST method must contain a hidden input whose name is specified by $csrfParam. You may use yii\helpers\Html::beginForm() to generate his hidden input.

The above code may be rewritten this way :

namespace app\controllers;

use Yii;
use yii\web\Controller;
use yii\filters\ContentNegotiator;
use yii\web\Response;

class UserController extends Controller
{
    public $enableCsrfValidation = false;

    public function behaviors()
    {
        return [
            'contentNegotiator' => [
                'class' => ContentNegotiator::className(),
                'formats' => [
                    'application/json' => Response::FORMAT_JSON,
                ],
                'only' => ['register'],
            ],
        ];
    }

    public function actionRegister()
    {
        $username = Yii::$app->request->post('username');
        return $username;
    }
}