<?php
/*
Plugin Name: Parent Page Resolver
Plugin URI: http://wordpress.org/#
Description: This plugin allows to retrieve the highest level parent page of a subpage.
Author: Kevin Wetzels
Version: 1.0
Author URI: http://www.el73.be/drunkey-love/
*/ 

function get_parent_page_of($subPageId) {
	if (!is_numeric($subPageId)) {
		echo 'The subpage id should be a number';
		return 0;
	} 
	global $wpdb;
	global $table_prefix;
	$parentPageId = $subPageId;
	$currentParentPageId = $subPageId;
	while ($currentParentPageId != 0) {
		$parentPageId = $currentParentPageId;
		$query = 'SELECT post_parent FROM ' . $table_prefix . 'posts WHERE id = ' . $currentParentPageId;			
		foreach ($wpdb->get_row($query, ARRAY_A) as $row ) {
			$currentParentPageId = $row['post_parent'];
		}
	}
	return $parentPageId;
}

?>