Get Related Pages Based on Menu Hierarchy in WordPress

relatedPages.php

<?php 
function getRelatedPages($pageID, $limit = 10) {
  // if this page has a parent, query for sybling pages
  // else if this page is a parent, query for child pages
  // else query for other top level pages
  $parentID = wp_get_post_parent_id(get_the_ID());
  $pages = [];
  if ($parentID) {
    $pages = Helpers::getPagesHierarchy([
      'sort_column' => 'menu_order',
      'child_of' => $parentID,
      'echo' => false
    ]);
  } else {
    $pages = Helpers::getPagesHierarchy([
      'sort_column' => 'menu_order',
      'parent' => get_the_ID(),
      'echo' => false
    ]);
  }
  if (!$pages) {
    $pages = Helpers::getPagesHierarchy([
      'sort_column' => 'menu_order',
      'parent' => 0,
      'echo' => false
    ]);
  }
  $pages = array_filter($pages, function($page) {
    return $page->ID !== get_the_ID();
  });
  $pages = array_slice($pages, 0, $limit);
  // if there are less than $limit pages, fill the rest with random pages not in the list
  if (count($pages) < $limit) {
    $randomPages = Helpers::getPagesHierarchy([
      'sort_column' => 'menu_order',
      'parent' => 0,
      'echo' => false
    ]);
    $randomPages = array_filter($randomPages, function($page) use ($pages) {
      return $page->ID !== get_the_ID() && !in_array($page->ID, array_map(function($page) {
        return $page->ID;
      }, $pages));
    });
    shuffle($randomPages);
    $pages = array_merge($pages, array_slice($randomPages, 0, $limit - count($pages)));
  }
  return $pages;
}