我认为您需要的是：

<?php 
$post_id = $post->ID; //or somehow get the current post ID.
$ancestors = get_post_ancestors($post_id)  
//$ancestors is an array of post IDs starting with the current post going up the root
//\'Pop\' the root ancestor out or returns the current ID if the post has no ancestors.
$root_id = (!empty($ancestors) ? array_pop($ancestors): $post_id);