WordPressの固定ページにおける親ページ判定関数

get_root_page.php

<?php
function get_root_page ( $cur_post, $cnt = 0 ) {
     if ( $cnt > 100 ) { return false; }
     $cnt++;
     if ( $cur_post->post_parent == 0 ) {
          $root_page = $cur_post;
     } else {
          $root_page = get_root_page( get_post( $cur_post->post_parent ), $cnt );
     }
     return $root_page;
}

$root_page = get_root_page($post);
$root_id = $root_page->ID;