PHP: WordPress return top most parent ID

get-earliest-relative.php

<?php
/**
 * [Gets the top most parent of a post. If post is top most parent, returns ID]
 * @return int [ID of top most parent]
 */
function get_earliest_relative($post){
    if ($post->post_parent){
        $ancestors=get_post_ancestors($post->ID);
        $root=count($ancestors)-1;
        $parent = $ancestors[$root];
    } else {
        $parent = $post->ID;
    }
    return $parent;
}