The UserProviderInterface interface - http://www.symfonycentral.com/securing-your-web-application-with-symfony.html

UserProviderInterface.php

<?php
interface UserProviderInterface
{
    /**
     * Loads the user object for the given username.
     *
     * @param string $username The username
     * @return UserInterface
     * @throws UsernameNotFoundException If the user is not found
     */
    function loadUserByUsername($username);

    /**
     * Refreshes the user for the account interface.
     *
     * @param UserInterface $user
     * @return UserInterface
     * @throws UnsupportedUserException If the user type is not supported
     */
    function refreshUser(UserInterface $user);

    /**
     * Whether this provider supports the given user class
     *
     * @param string $class
     * @return Boolean true if the provided user object is supported by this implementation of UserProviderInterface
     */
    function supportsClass($class);
}