// src/AppBundle/Form/UserType.php
namespace AppBundle\Form;

use AppBundle\Entity\User;
use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class UserType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('email', 'email')
            ->add('username', 'text')
            ->add('plainPassword', 'repeated', array(
                'type' => 'password',
                'first_options'  => array('label' => 'Password'),
                'second_options' => array('label' => 'Repeat Password'),
            ))
        ;
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults(array(
            'data_class' => User::class,
        ));
    }

    public function getName()
    {
        return 'user';
    }
}

// src/AppBundle/Controller/RegistrationController.php
namespace AppBundle\Controller;

use AppBundle\Form\UserType;
use AppBundle\Entity\User;
use Sensio\Bundle\FrameworkExtraBundle\Configuration\Route;
use Symfony\Bundle\FrameworkBundle\Controller\Controller;
use Symfony\Component\HttpFoundation\Request;

class RegistrationController extends Controller
{
    /**
     * @Route("/register", name="user_registration")
     */
    public function registerAction(Request $request)
    {
        // 1) build the form
        $user = new User();
        $form = $this->createForm(new UserType(), $user);

        // 2) handle the submit (will only happen on POST)
        $form->handleRequest($request);
        if ($form->isSubmitted() && $form->isValid()) {

            // 3) Encode the password (you could also do this via Doctrine listener)
            $password = $this->get('security.password_encoder')
                ->encodePassword($user, $user->getPlainPassword());
            $user->setPassword($password);

            // 4) save the User!
            $entityManager = $this->getDoctrine()->getManager();
            $entityManager->persist($user);
            $entityManager->flush();

            // ... do any other work - like sending them an email, etc
            // maybe set a "flash" success message for the user

            return $this->redirectToRoute('replace_with_some_route');
        }

        return $this->render(
            'registration/register.html.twig',
            array('form' => $form->createView())
        );
    }
}

// src/AppBundle/Entity/User.php
// ...

class User implements UserInterface
{
    // ...

    public function getUsername()
    {
        return $this->email;
    }

    // ...
}

// src/AppBundle/Form/UserType.php
// ...
use Symfony\Component\Validator\Constraints\IsTrue;

class UserType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder
            ->add('email', 'email');
            // ...
            ->add('termsAccepted', 'checkbox', array(
                'mapped' => false,
                'constraints' => new IsTrue(),
            ))
        );
    }
}