include($forum_root) from root does not output, yet include($forum_http) does..

include($forum_root) from root does not output..
$forum_root='/home/www/foobar/v1/bin/forum3/index.php';

yet include($forum_http) does..
$forum_http='http://foobar.com/v1/bin/forum3/index.php';

my question, $forum_root should work.. yet there's something stopping it from outputting.. Though, what exactly is stopping it?

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Those questioning, I'm producing a gateway (not portal)..
I have successfully created one via http and transmitting the vars needed by add a query string or db. This is because include($forum_http); loses global scopes. Yet now that I know it works. I rather include via root and have global scope to transmit the data.