An Entity class for a User - http://www.symfonycentral.com/symfony-does-validation.html

User.php

<?php
namespace MyCompany\BlogBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/* Include the required validators */
use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
use Symfony\Component\Validator\Constraints as Assert;


/* Note below: we can make assertions at the class / entity level */

/**
 * MyCompany\BlogBundle\Entity\User
 *
 * @ORM\Table()
 * @ORM\Entity(repositoryClass="MyCompany\BlogBundle\Entity\UserRepository")
 *
 * @UniqueEntity("email")
 * @UniqueEntity("username")
 */
class User
{
    /**
     * @var string $username
     *
     * @ORM\Column(name="username", type="string", length=255)
     *
     * @Assert\MinLength(
     *     limit=5,
     *     message="Username must have at least {{ limit }} characters."
     * )
     * @Assert\MaxLength(
     *     limit=50,
     *     message="Username must have at most {{ limit }} characters."
     * )
     */
    private $username;

    /**
     * @var string $email
     *
     * @ORM\Column(name="email", type="string", length=255)
     *
     * @Assert\Email(
     *     message = "The value '{{ value }}' is not a valid email."
     * )
     */
    private $email;

    /**
     * @var string $password
     *
     * @ORM\Column(name="password", type="string", length=31)
     *
     * @Assert\MinLength(
     *     limit=5,
     *     message="Password must have at least {{ limit }} characters."
     * )
     * @Assert\MaxLength(
     *     limit=50,
     *     message="Password must have at most {{ limit }} characters."
     * )
     */
    private $password;

    /**
     * @var integer $id
     *
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    private $id;
    
    /* 
     * Getters and setters omitted for brevity... 
     */
}