DefaultController.php

<?php

namespace APIBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Symfony\Component\HttpFoundation\JsonResponse;

class DefaultController extends Controller
{
    /**
     * @Route("/")
     */
    public function indexAction()
    {
        return new JsonResponse(['msg' => 'Serie Symfony 3.4 API']);
    }
}