walker.php

<?php
class My_Walker_Nav_Menu extends Walker_Nav_Menu {
  function start_lvl(&$output, $depth) {
    $indent = str_repeat("\t", $depth);
    $output .= "\n$indent<ul class=\"dropdown-menu\">\n";
  }
  function start_el( &$output, $item, $depth, $args ) {
        if ( $args->has_children ) {
            // ...
        }
    }
}

add_filter('nav_menu_css_class', 'my_css_attributes_filter', 100, 1);
        add_filter('nav_menu_item_id', 'my_css_attributes_filter', 100, 1);
        function my_css_attributes_filter($var) {
            if(is_array($var)){
                $varci= array_intersect($var, array('current-menu-item'));
                $cmeni = array('current-menu-item');
                $selava   = array('active');
                $selavaend = array();
                $selavaend = str_replace($cmeni, $selava, $varci);
            }
            else{
                $selavaend= '';
            }
        return $selavaend;
        } 

?>