How to config symfony to log deprecations?

Question

I'd like to upgrade from Symfony 4.4. to 5.0. So I have to check for deprecations in the code. The symfony migration guide says I have to use the web dev toolbar, but in my API-app there is no frontend for the tool-bar.

How can I configure symfony/monolog to log deprecation warnings to the log file?

Update I have created a minimal example:

 composer create-project symfony/website-skeleton:4.3.99

TestController.php

<?php

namespace App\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\AbstractController;
use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\Routing\Annotation\Route;

/**
 * Class ApiController
 * @package App\Controller
 * @Route("", defaults={"_format"="json"})
 *
 */
class TestController extends AbstractController
{

    /**
     * @Route("/test", name="test")
     * @param Request $request
     * @return Response
     */
    public function test(Request $request): Response
    {
        @trigger_error(sprintf('DEMO DEPRECATION', __METHOD__), E_USER_DEPRECATED);
        return $this->json([
            'test' => '1'
        ]);
    }
}

monolog.yml

monolog:
    handlers:
        main:
            type: stream
            path: "%kernel.logs_dir%/%kernel.environment%.log"
            level: debug
            channels: ["!event"]
        # uncomment to get logging in your browser
        # you may have to allow bigger header sizes in your Web server configuration
        #firephp:
        #    type: firephp
        #    level: info
        #chromephp:
        #    type: chromephp
        #    level: info
        console:
            type: console
            process_psr_3_messages: false
            channels: ["!event", "!doctrine", "!console"]
        deprecation_stream:
              type: stream
              path: "%kernel.logs_dir%/%kernel.environment%.deprecations.log"
        deprecation_filter:
             type: filter
             handler: deprecation_stream
             max_level: info
             channels: ["php"]

run server

 bin/console server:run

open http://localhost/test

But the dev.deprecations.log is still empty.

Answer 1

There was a bug in Symfony 4.4 which is now fixed. More details here.