Using a custom UserInterface for Symfony Security - http://www.symfonycentral.com/securing-your-web-application-with-symfony.html

User.php

<?php

namespace MyCompany\BlogBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

// Add this so that we load the UserInterface class
use Symfony\Component\Security\Core\User\UserInterface;

class User implements UserInterface // Make sure we implement UserInterface
{

    /* Auto-generated code... */

    function getRoles()
    {
        // We aren't going to use roles for this demonstration.
        return array();
    }

    function eraseCredentials()
    {
        // We're going to be using hashed passwords, so we don't have to do anything here.
        return;
    }

    function equals(UserInterface $user)
    {
        // We're going to use 'username' as the primary identifier, so let's use that for comparison...
        return $this->getUsername() == $user->getUsername();
    }
}