VendorAppBundlerControllerDefaultController.php

<?php

// ...

$user = new User;
$form = $this->createFormBuilder($user)
    ->add('email', 'email')
    ->getForm();

if ($request->getMethod() == 'POST') {
    $em = $this->get('doctrine')->getEntityManager();
    $form->bindRequest($request);
    if ($form->isValid()) {
        $em = $this->get('doctrine')->getEntityManager();
        $em->persist($user);
        $em->flush();
        // ...
    }

// ...

VendorAppBundlerEntityUser.php

<?php

// ...

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\Validator\Constraints;
use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;

/**
 * @ORM\Entity
 * @UniqueEntity("email")
 */
class User {

    // ...

    /**
     * @Constraints\Email
     * @ORM\Column(type="string", length=255, unique=true)
     */
    private $email;

    function setEmail($email) {
        $this->email = $email;
    }

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