accessing global variables

WordPress is installed at https://www.mydomain.com/wordpress/

On the comments.php page (which shows up on https://www.mydomain.com/wordpress/permalink/, I added the code:

<?php global $user_ID; echo('User ID: ' . $user_ID); ?>

Which, when a user is logged in, properly displays their User ID.

I have another page at https://www.mydomain.com/somewhere_else/
I added the code:

<?php global $user_ID; echo('User ID: ' . $user_ID); ?>

This does not work as it did above. I presume it is because it is on a page not within the wordpress installation.

How can I get it to work?

Thanks,
~marc