Using $vbulletin or $db in non-vb powered php file?

[saajjj]

Hello,
This post seems to have grown. I wanted to explain exactly what I was doing so that someone might point me in the right direction.

I've made a custom template inside which I want to output from a PHP file.
This customTemplate is itself used inside the header template. The plugin for this customTemplate looks like:
(hook: parse_templates)

$templater = vB_Template::create('customTemplate');
$templatevalues['customTemplate_show'] = $templater->render();
vB_Template::preRegister('header', $templatevalues);

The plugin I'm using to enable me to write from the PHP file is:
(hook: parse_templates)

ob_start();
	include("http://www.myforum.com/myPHPfile.php");
	$contents = ob_get_contents();
ob_end_clean();
$preRegister['phpFile_output'] = $contents;
vB_Template::preRegister('customTemplate', $preRegister);

So, the forum header has {vb:raw customTemplate_show} and customTemplate has {vb:raw phpFile_output}

The PHP file is simply:

<?php
	echo("hello world");
?>

The above works as expected. i.e in the header I can see 'hello world'.

My problem starts when I want to start using vB specific globals in myPHPFile.php like $vbulletin or $db. I get a:
Fatal error: Call to a member function query_read() on a non-object in /home/xxxx/public_html/myPHPFile.php on line 9

where line 9 is:

$result = $db->query_read("Select * from ".TABLE_PREFIX."myTable");

My understanding is that this error means that $db is not an object that PHP recognizes. This led me to try out various things, none of which worked. I tried adding the following to the start of myPHPfile.php:
1. require_once("./global.php"); <--- this starts an infinite loop, and eventually crashes the browser
2. global $vbulletin
3. global $db

I'd like to point out that, thanks to articles posted here, I have been able to use $vbulletin and $db in a regular vb powered custom page without any issues.

Any help appreciated.