class megaMenuWalker extends Walker_Nav_Menu {
        function start_lvl(&$output, $depth = 0, $args = array()) {
            $indent = str_repeat("\t", $depth);
            if ($depth == 0) {
                $output .= "\n$indent<section>\n";
            }
            $output .= "<ul class=\"sub-menu\">\n";
        }
        function end_lvl(&$output, $depth = 0, $args = array()) {
            $indent = str_repeat("\t", $depth);
            $output .= "\n$indent</ul>\n";
            if ($depth == 0) {
                $output .= "</section>\n";
            }
        }
    }