Resolve the error “Use of undefined constant DS” of require_once method in Joomla 3.0

In Joomla 1.5 to 2.5 we include directly the helper file or other file in the Joomla PHP code using require_once() method but in Joomla 3.0 you must defined the DS directory separator constant otherwise you getting error message.

defined('_JEXEC') or die('Restricted access');
require_once (dirname(__FILE__).DS.'helper.php');

Here is the right way to import your file using the DIRECTORY_SEPARATOR constant

defined('_JEXEC') or die('Restricted access');
if(!defined('DS')){
    define('DS',DIRECTORY_SEPARATOR);
}
require_once (dirname(__FILE__).DS.'helper.php');