How do I set a $_REQUEST variable so that an included vB script will use it?

[Spinball]

I am trying to display a thread (minus the header/footer and some other bits) in a webpage on the same server as my vB installation.
I have created a cut-down version of SHOWTHREAD template called SHOWTHREAD_inline.
I have made a copy of showthread.php to showthread_inline.php and changed it to use the SHOWTHREAD_inline template.
I have then included showthread_inline.php into my custom php script.

It works but just shows the 'no thread specified' error. I can't set the ID of the thread I want to display.
I have tried :

PHP Code:

$_REQUEST['t'] = 123456;
include_once("path/to/showthread_inline.php"); 


but the value of t is not taken into the included code.

How do I do this, please?
Thanks