I am indepted to Paul S. Owen, Development Team Leader of phpBB, the standard forum for PHP-Nuke starting from the later 6.x versions, for giving permission to include the following explanation, which can be found in http://www.phpbb.com/phpBB/viewtopic.php?t=69493, as well as in http://www.phpbb.com/kb/article.php?article_id=54: