Why am I having issues deploying Symfony 4 app to Heroku?

In my case is I did not add the route "/". Symfony load default page in local environment without that route, but in heroku we need to add it!

I show my code for your reference:

class DefaultController extends AbstractController{    /**     * @Route("/", name="default")     */    public function ping()    {        return $this->json([            'message' => 'pong'        ]);    }}

In case anyone else stumbles upon this trying to deploy a Symfony / Symfony 4 app, these are the steps I had to take to solve the issue:

• My .htaccess file was outdated and still referenced the old Symfony 3 app.php. What I had to do was delete it and get the new one via composer require symfony/apache-pack
• At this point, I still got a 500 Error and the Heroku logs weren't saying anything helpful. That's because Heroku requires you to log things to php://stderr to have them properly logged in the CLI and web interface. As such, I had to modify my config/packages/prod/monolog.yaml file to have the nested handler be like this: path: "php://stderr"

In my case, the second issue turned out being I'd forgotten to run node_modules/.bin/encore production in my postinstall script - but just redirecting logging to php://stderr should point anyone in the right direction.