Forget about this comment. I force installed this version again and 
now REASON_LOCKED is defined. I think this can be closed. Sorry about 
the noise.

Not in the version I have installed:

# pear info horde/Horde_Auth
About pear.horde.org/Horde_Auth-1.4.8
=====================================
Release Type          PEAR-style PHP-based Package
Name                  Horde_Auth (extends Auth)
Channel               pear.horde.org
Summary               Horde Authentication API
Description           The Horde_Auth package provides a common
                       interface into the various backends for the
                       Horde authentication system.
Maintainers           Chuck Hagenbuch <chuck@horde.org> (lead)
                       Jan Schneider <jan@horde.org> (lead)
                       Michael Slusarz <slusarz@horde.org> (lead)
Release Date          2012-01-31 11:23:54
Release Version       1.4.8 (stable)

[...]

# cat -n /usr/share/php5/PEAR/Horde/Auth.php

      1  <?php
      2  /**
      3   * The Horde_Auth:: class provides a common abstracted 
interface for various
      4   * authentication backends.  It also provides some useful
      5   * authentication-related utilities.
      6   *
      7   * Copyright 1999-2011 The Horde Project (http://www.horde.org/)
      8   *
      9   * See the enclosed file COPYING for license information 
(LGPL). If you did
     10   * not receive this file, see 
http://opensource.org/licenses/lgpl-2.1.php
     11   *
     12   * @author   Chuck Hagenbuch <chuck@horde.org>
     13   * @author   Michael Slusarz <slusarz@horde.org>
     14   * @category Horde
     15   * @license  http://opensource.org/licenses/lgpl-2.1.php LGPL
     16   * @package  Auth
     17   */
     18  class Horde_Auth
     19  {
     20      /**
     21       * Authentication failure reasons.
     22       *
     23       * <pre>
     24       * REASON_BADLOGIN - Bad username and/or password
     25       * REASON_FAILED - Login failed
     26       * REASON_EXPIRED - Password has expired
     27       * REASON_LOGOUT - Logout due to user request
     28       * REASON_MESSAGE - Logout with custom message
     29       * REASON_SESSION - Logout due to session expiration
     30       * </pre>
     31       */
     32      const REASON_BADLOGIN = 1;
     33      const REASON_FAILED = 2;
     34      const REASON_EXPIRED = 3;
     35      const REASON_LOGOUT = 4;
     36      const REASON_MESSAGE = 5;
     37      const REASON_SESSION = 6;
     38

That's not true, REASON_LOCKED *is* in Horde_Auth 1.4.8.
We still mustn't rely on it though.

When installing the latest (stable) pear, the Horde_Auth package seems 
to be out-of-date and results in a fatal PHP error when trying to login:

PHP Fatal error:  Undefined class constant 'REASON_LOCKED' in 
/srv/www/htdocs/horde4/login.php on line 335

The REASON_LOCKED constant is available in the development version of 
Horde_Auth, but hasn't made it into the latest stable version (1.4.8) 
on pear.