Monolog ErrorHandler in Symfony2 apps

AppKernel.php

<?php

use Monolog\ErrorHandler;
use Psr\Log\LogLevel;
use Psr\Log\LoggerInterface;
use Symfony\Component\HttpKernel\Kernel;
use Symfony\Component\DependencyInjection\ContainerInterface;

class AppKernel extends Kernel
{
    public function boot()
    {
        if (true === $this->booted) {
            return;
        }

        parent::boot();

        $logger = $this->getContainer()->get('logger', ContainerInterface::NULL_ON_INVALID_REFERENCE);
        if ($logger instanceof LoggerInterface) {
            ErrorHandler::register($logger, [], LogLevel::ERROR, LogLevel::ALERT);
        }
    }
}