Simple function for creating a subnav in your Wordpress theme

wp-simple-subnav.php

<?php
/**
 * author: Robert Danklefsen
 * website: https://www.catchylabs.com/
 *
 * Simple function for creating a subnav in your Wordpress theme
 */
 
function catchylabs_get_top_parent_page_id() {
    global $post;
    $ancestors = $post->ancestors;
    // Check if page is a child page (any level)
    if ($ancestors) {
        //  Grab the ID of top-level page from the tree
        return end($ancestors);
    } else {
        // Page is the top level, so use  it's own id
        return $post->ID;
    }
}

$ancestor_id = catchylabs_get_top_parent_page_id();
$children = get_pages(array('child_of' => $post->post_parent, 'parent' => $post->post_parent, 'sort_order' => 'asc', 'sort_column' => 'menu_order'));

if ($children) :
?>

<div class="subnav right">
  <ul>
    <?php foreach ($children as $child) : ?>
    <li class="<?= ($child->ID == $post->post_parent) ? "current_page_item" : ""; ?> <?= ($child->ID == $post->ID) ? "current_page_item" : ""; ?>">
      <a href="<?= get_the_permalink($child->ID) ?>"><?= $child->post_title ?></a>
    </li>
    <?php endforeach; ?>
  </ul>
  <div class="mobile-select">
    <select onchange="document.location.href=this.options[this.selectedIndex].value;">
      <option value="-1">- More Products -</option>
      <?php foreach ($children as $child) : ?>
        <option value="<?= get_the_permalink($child->ID) ?>"><?= $child->post_title ?></option>
      <?php endforeach; ?>
    </select>
  </div>
</div>

<?php endif; ?>