AppKernel.php

<?php

use Symfony\Component\HttpKernel\Kernel;

class AppKernel extends Kernel
{
    public static function fromEnvironment()
    {
        $env = getenv('APP_ENVIRONMENT') ?: 'prod';
        $debug = filter_var(getenv('APP_DEBUG'), FILTER_VALIDATE_BOOLEAN);

        return new self($env, $debug);
    }
     // ...
}

index.php

<?php

use Symfony\Component\HttpFoundation\Request;

// require autoloader or bootstrap
require __DIR__ . '/../app/autoload.php';

// from the environment!
$app = AppKernel::fromEnvironment();

// just like normal
$request = Request::createFromGlobals();
$response = $app->handle($request);
$response->send();
$app->terminate($request, $response);