index.php

<?php

$loader = new Phalcon\Loader();

$loader->registerNamespaces(array(
   'Application\Backend\Controllers' => '../app/backend/controllers/',
   'Application\Frontend\Controllers' => '../app/frontend/controllers/',
))->register();

$di = new \Phalcon\DI\FactoryDefault();

$di->set('router', function() {

	$router = new \Phalcon\Mvc\Router\Annotations(false);

	$router->addModuleResource('frontend', 'Application\Backend\Controllers\Products', '/api/products');

	return $router;
});

$application = new \Phalcon\Mvc\Application();

$application->setDI($di);

$application->registerModules(array(
	'frontend' => //...
));

try {
	echo $application->handle()->getContent();
} catch (Exception $e) {
	var_dump($e->getMessage());
}