php__silex__book/src/demo/UserConverter.php

<?php

namespace Demo;

use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;

class UserConverter {

  public function convert($id) {
    $id = (int) $id;
    if ($id <= 0) {
      throw new NotFoundHttpException(sprintf('User not found'));
    }

    return new User($id);
  }
}