Get the ID of the "Root Resource", aka "Ultimate Parent", with slightly different feature set.

rootResource.php

<?php

/*
 * @author @sepiariver
 *
 * GPL license, no warranties, no liability, etc.
 *
 * USAGE EXAMPLE: 
 * [[rootResource? &toPlaceholder=`root_resource`]]
 * //followed by something like
 * [[getResources? &parents=`[[+root_resource]]` ... ]]
 *
 */
 
$id = $modx->getOption('id', $scriptProperties, $modx->resource->get('id'));
$toPlaceholder = $modx->getOption('toPlaceholder', $scriptProperties, '');

$pids = array_reverse($modx->getParentIds($id));
$output = (isset($pids[1]) && !empty($pids[1])) ? $pids[1] : $id;

if (empty($toPlaceholder)) return $output;
$modx->setPlaceholder($toPlaceholder, $output);