WordPress Custom Menus classes

custom-menus-classes.php

<?php
add_filter('nav_menu_css_class', 'normalize_wp_classes', 10, 2);

// for the page menu fallback (wp_list_pages)
add_filter('page_css_class', 'normalize_wp_classes', 10, 2);

function normalize_wp_classes($classes, $item){

  // old class => new class
  $replacements = array(
    'current-menu-item'     => 'active',
    'current-menu-parent'   => 'active-parent',
    'current-menu-ancestor' => 'active-parent',
    'current_page_item'     => 'active',
    'current_page_parent'   => 'active-parent',
    'current_page_ancestor' => 'active-parent',
    'current-page-item'     => 'active',
    'current-page-parent'   => 'active-parent',
    'current-page-ancestor' => 'active-parent'
  );

  // do the replacements above
  $classes = strtr(implode(',', $classes), $replacements);
  $classes = explode(',', $classes);

  // remove any classes that are not present in the replacements array,
  // and return the result
  return array_unique(array_intersect(array_values($replacements), $classes));
}