get the topmost parent of a wordpress page

get_topmost_parent.php

<?php
// recursion is our friend here

function get_topmost_parent($post_id){
  $parent_id = get_post($post_id)->post_parent;
  if($parent_id == 0){
    return $post_id;
  }else{
    return get_topmost_parent($parent_id);
  }
}
$parent = get_post(get_topmost_parent($post->id));

?>