form.yml

services:
    form.handler:
       class: JStout\MainBundle\Component\Form\FormHandler
       scope: request
       arguments: [@request, @doctrine.orm.entity_manager]

FormHandler.php

<?php

namespace JStout\MainBundle\Component\Form;

use Symfony\Component\Form\Form,
    Doctrine\ORM\EntityManager,
    Symfony\Component\HttpFoundation\Request,
    JStout\MainBundle\Component\Form\FormHandlerInterface;

class FormHandler
{
    protected $request;
    protected $entityManager;

    public function __construct(Request $request, EntityManager $entityManager)
    {
        $this->request = $request;
        $this->entityManager = $entityManager;
    }

    public function create(FormHandlerInterface $handler, Form $form)
    {
        $handler->buildFormHandler($form, $this->request, $this->entityManager);

        return $handler;
    }
}

FormHandlerInterface.php

<?php

namespace JStout\MainBundle\Component\Form;

use Symfony\Component\Form\Form,
    Symfony\Component\HttpFoundation\Request,
    Doctrine\ORM\EntityManager;

interface FormHandlerInterface
{
    public function buildFormHandler(Form $form, Request $request, EntityManager $entityManager);
}

SignupController.php

<?php

namespace JStout\MainBundle\Controller;

use Symfony\Bundle\FrameworkBundle\Controller\Controller,
    Sensio\Bundle\FrameworkExtraBundle\Configuration\Route,
    Sensio\Bundle\FrameworkExtraBundle\Configuration\Template,
    JStout\MainBundle\Entity\User,
    JStout\MainBundle\Form\User\SignupType,
    JStout\MainBundle\Form\User\SignupHandler;

/**
 * @Route("/user")
 */
class UserController extends Controller
{
    /**
     * @Route(name="user")
     * @Template()
     */
    public function indexAction()
    {
        return array();
    }

    /**
     * @Route("/sign-up", name="user_signup")
     * @Template()
     */
    public function signupAction()
    {
        $user = new User();
        $form = $this->get('form.factory')->create(new SignupType(), $user);
        $formHandler = $this->get('form.handler')->create(new SignupHandler(), $form);
        
        if ($formHandler->process($user, $this->container->get('security.encoder_factory')->getEncoder($user))) {
            return $this->redirect($this->generateUrl('success'));
        }

        return array(
            'form' => $form->createView()
        );
    }
}

SignupHandler.php

<?php

namespace JStout\MainBundle\Form\User;

use JStout\MainBundle\Component\Form\FormHandlerInterface,
    Symfony\Component\Form\Form,
    Symfony\Component\HttpFoundation\Request,
    Doctrine\ORM\EntityManager,
    JStout\MainBundle\Entity\User;

class SignupHandler implements FormHandlerInterface
{
    protected $form;
    protected $request;
    protected $entityManager;

    public function buildFormHandler(Form $form, Request $request, EntityManager $entityManager)
    {
        $this->form = $form;
        $this->request = $request;
        $this->entityManager = $entityManager;
    }

    public function process(User $user, $encoder)
    {

        if ('POST' == $this->request->getMethod()) {
            $this->form->bindRequest($this->request);
            
            if ($this->form->isValid()) {
                $user->setSalt(base_convert(sha1(uniqid(mt_rand(), true)), 16, 36));
                $user->setPassword($encoder->encodePassword($user->getPlainPassword(), $$user->getSalt()));
                
                $this->entityManager->persist($user);
                $this->entityManager->flush();
                
                return true;
            }
        }

        return false;
    }
}

SignupType.php

<?php

namespace JStout\MainBundle\Form\User;

use Symfony\Component\Form\AbstractType,
    Symfony\Component\Form\FormBuilder;

class SignupType extends AbstractType
{
    public function buildForm(FormBuilder $builder, array $options)
    {
        $builder
            ->add('firstName')
            ->add('lastName')
            ->add('email')
            ->add('plainPassword', 'repeated', array('type' => 'password'));
    }
}