Quick infinite-depth subnav for WordPress

section_nav.php

<?php

function cad_section_nav($title = true) {
  global $post;

  if (!is_object($post)) return;

  $section = get_post_ancestor($post->ID);
  $pages = get_pages(array(
    'parent' => $section->ID
    )
  );

  $sectionPermalink = get_permalink($section);
  if ($title) echo "<a href=\"{$sectionPermalink}\">{$section->post_title}</a>";
  echo pages_as_list($pages, $post);
}

function pages_as_list($pages, $currentPage) {
  if (!count($pages)) return false;
  $output = "<ul>";

  foreach ($pages as $item) {
    $permalink = get_permalink($item->ID);
    $children  = get_pages(array('parent' => $item->ID));
    $active    = post_is_ancestor_of($currentPage, $item) || $item->ID == $currentPage->ID;
    $class     = ($active ? 'active' : '');

    if (count($children) && $active) {
      $output .= "<li class=\"{$class}\">";
      $output .=   "<a href=\"{$permalink}\">{$item->post_title}</a>";
      $output .= pages_as_list($children, $currentPage);
      $output .= "</li>";
    } else {
      $output .= "<li class=\"{$class}\"><a href=\"{$permalink}\">{$item->post_title}</a></li>";
    }
  }

  $output .= "</ul>";
  return $output;
}

function get_post_ancestor($postID) {
  $post = get_post($postID);

  if (!is_object($post)) return;
  while ($post->post_parent != 0): $post = get_post($post->post_parent); endwhile;
  return $post;
}


function post_is_ancestor_of($post, $ancestor) {
  if (!is_object($post) || !is_object($ancestor)) return;

  $ancestors = get_post_ancestors($post);
  return (in_array($ancestor->ID, $ancestors));
}