Symfony2 : Authentication with Active directory
class LdapUser implements UserInterface
{
    private $username;
    private $first_name;
    private $last_name;
    private $password;
    private $salt;
    private $roles;

    public function __construct($username, $first_name, $last_name, $password, $salt, array $roles) {
        $this->username = $username;
        $this->first_name = $first_name;
        $this->last_name = $last_name;
        $this->password = $password;
        $this->salt = $salt;
        $this->roles = $roles;
    }

    ...

}

public function loadUserByUsername($username)
{

    $server = "my_ldap_server";
    $root_dn = "my_root_dn";
    $root_pw = "my_root_pw";

    $ds = ldap_connect($server);
    if ($ds) {

        ldap_bind($ds, $root_dn, $root_pw);
        $search = ldap_search($ds, "my_branch", "(sAMAccountName=".$username.")", array("sn", "givenName"));
        $info = ldap_get_entries($ds, $sr);

        if($info['count'] > 0) {
            $user = $info[0];
            return new LdapUser($username, $user['givenName'][0], $user['sn'][0], '???PASSWORD???', '???SALT???', array('ROLE_USER'));
        } else {
            throw new UsernameNotFoundException(sprintf('Username "%s" does not exist.', $username));
        }

        ldap_close($ds);
    } else {
        echo "Connexion au serveur LDAP impossible";
    }
}