Below is a copy of my file :

includes/languages/english/header.php

PHP Code:
<?php
/**
 * @package languageDefines
 * @copyright Copyright 2003-2006 Zen Cart Development Team
 * @copyright Portions Copyright 2003 osCommerce
 * @license http://www.zen-cart.com/license/2_0.txt GNU Public License V2.0
 * @version $Id: header.php 2940 2006-02-02 04:29:05Z drbyte $
 */

// header text in includes/header.php
  
define('HEADER_TITLE_CREATE_ACCOUNT''Create Account');
  
define('HEADER_TITLE_MY_ACCOUNT''My Account');
  
define('HEADER_TITLE_CART_CONTENTS''Shopping Cart');
  
define('HEADER_TITLE_CHECKOUT''Checkout');
  
define('HEADER_TITLE_TOP''Top');
  
define('HEADER_TITLE_CATALOG''Home');
  
define('HEADER_TITLE_LOGOFF''Log Out');
  
define('HEADER_TITLE_LOGIN''Log In');

// added defines for header alt and text
  
define('HEADER_ALT_TEXT''Powered by Zen Cart :: The Art of E-Commerce');
  
define('HEADER_SALES_TEXT''TagLine Here');
  
define('HEADER_LOGO_WIDTH''192px');
  
define('HEADER_LOGO_HEIGHT''64px');
  
define('HEADER_LOGO_IMAGE''logo.gif');

// header Search Button/Box Search Button
  
define('HEADER_SEARCH_BUTTON','Search');
  
define('HEADER_SEARCH_DEFAULT_TEXT''Enter search keywords here');

?>
For some reason, you've got:-

/*define('HEADER_SEARCH_DEFAULT_TEXT', 'Enter search keywords here');*/?>

Not sure why an attempt has been made to comment out the line - and besides, this is not how tyou comment out a line in PHP - but is the method for CSS.