How to get the current page id without executing the handleRequest method?

<?php
include 'path/to/modx/class';
$modx = new modX();


<!-- Other core code and paths -->

$modx->initialize('web');

include 'page/that/needs/the/current/page/resource'; // The resource id is needed here.

$modx->handleRequest(); // This will expose the resource id in the $modx object, but renders the page as well which is not intended in this case.
?>

Simple example:

$modx = new modX();
$modx->initialize('web');

if($_POST['comment'])
{
   $currentDoc = $modx->documentIdentifier;  // NULL. How to get access at this state to the document resource.
   save_data($_POST['newPage'], $currentDoc); 
}

$modx->handleRequest();