An example of using the core Symfony components outside of a framework.

symfony-bare.php

<?php
//
// Symfony Bare
//
// php -S localhost:8181 symfony-bare.php
//
// http://localhost:8181/is-leap-year
//

require_once 'vendor/autoload.php';

use Symfony\Component\Debug;
use Symfony\Component\DependencyInjection as DI;
use Symfony\Component\EventDispatcher;
use Symfony\Component\HttpFoundation;
use Symfony\Component\HttpKernel;
use Symfony\Component\Routing;

//
// Service
//
class LeapYearService
{
    public function isLeapYear($year = null)
    {
        if (null === $year) {
            $year = date('Y');
        }

        return 0 === $year % 400 || (0 === $year % 4 && 0 !== $year % 100);
    }
}

//
// Listener
//
class StringResponseListener implements EventDispatcher\EventSubscriberInterface
{
    public function onView(HttpKernel\Event\GetResponseForControllerResultEvent $event)
    {
        $result = $event->getControllerResult();
        if (is_string($result)) {
            $event->setResponse(new HttpFoundation\Response($result));
        }
    }

    public static function getSubscribedEvents()
    {
        return array('kernel.view' => 'onView');
    }
}

//
// Controllers
//
class LeapYearController
{
    private $leapYearService;
  
    public function __construct(LeapYearService $leapYearService)
    {
        $this->leapYearService = $leapYearService;
    }
  
    public function indexAction($year)
    {
        if ($this->leapYearService->isLeapYear($year)) {
            return 'Yep, this is a leap year!';
        }

        return 'Nope, this is not a leap year.';
    }
}

class ErrorController
{
    public function exceptionAction(Debug\Exception\FlattenException $exception)
    {
        return new HttpFoundation\Response(
            sprintf('Something went wrong! (%s)', $exception->getMessage()),
            $exception->getStatusCode()
        );
    }
}

//
// Routes
//
$routes = new Routing\RouteCollection();
$routes->add('leap_year', new Routing\Route('/is-leap-year/{year}', [
    'year'        => null,
    '_controller' => 'controller.leap_year:indexAction',
]));

//
// Container
//
$container = new DI\ContainerBuilder();

// Parameters
$container->setParameter('debug', true);
$container->setParameter('charset', 'UTF-8');
$container->setParameter('routes', $routes);

// My Services
$container->register('service.leap_year', LeapYearService::class);
$container->register('listener.string_response', StringResponseListener::class);
$container->register('controller.leap_year', LeapYearController::class)
    ->setArguments([new DI\Reference('service.leap_year')])
;
$container->register('controller.error', ErrorController::class);

// Symfony Services
$container->register('request_context', Routing\RequestContext::class);
$container->register('url_matcher', Routing\Matcher\UrlMatcher::class)
    ->setArguments(['%routes%', new DI\Reference('request_context')])
;
$container->register('request_stack', HttpFoundation\RequestStack::class);

$container->register('controller_resolver', HttpKernel\Controller\ContainerControllerResolver::class)
    ->setArguments([$container])
;
$container->register('argument_resolver', HttpKernel\Controller\ArgumentResolver::class);

$container->register('listener.router', HttpKernel\EventListener\RouterListener::class)
    ->setArguments([new DI\Reference('url_matcher'), new DI\Reference('request_stack')])
;
$container->register('listener.response', HttpKernel\EventListener\ResponseListener::class)
    ->setArguments(['%charset%'])
;
$container->register('listener.exception', HttpKernel\EventListener\ExceptionListener::class)
    ->setArguments(['controller.error:exceptionAction'])
;

$container->register('dispatcher', EventDispatcher\EventDispatcher::class)
    ->addMethodCall('addSubscriber', [new DI\Reference('listener.router')])
    ->addMethodCall('addSubscriber', [new DI\Reference('listener.string_response')])
    ->addMethodCall('addSubscriber', [new DI\Reference('listener.response')])
    ->addMethodCall('addSubscriber', [new DI\Reference('listener.exception')])
;

$container->register('kernel', HttpKernel\HttpKernel::class)
    ->setArguments([
        new DI\Reference('dispatcher'),
        new DI\Reference('controller_resolver'),
        new DI\Reference('request_stack'),
        new DI\Reference('argument_resolver'),
    ])
;

//
// Front Controller
//
$request  = HttpFoundation\Request::createFromGlobals();
$response = $container->get('kernel')->handle($request);
$response->send();

😍