Bootstrap menu - top level menu link active

/**
 * Add additional class for top level menu items
 *
 * @param string $item_output
 * @param obj $item
 * @param int $depth
 * @param obj $args
 * @return string
 */
function volcanno_top_menu_link_active($item_output, $item, $depth, $args){
    if($depth == 0){
        return str_replace('dropdown-toggle', 'dropdown-toggle disabled', $item_output);
    }else{
        return $item_output;
    }

}

add_filter('walker_nav_menu_start_el', 'volcanno_top_menu_link_active', 10, 4);