[Why can't I include a full url? (noob problem...)]

It's just a simple question...

 

<?php

include ('beginning.txt');

?>

 

Why can't I include the whole url, eg. ... include ('http://www.mywebsite.com/random/beginning.txt') ... ? It just shows a blank page if I type in the whole url name. I would like to include the file beginning.txt into several pages that are under the folder random. And another question... does it matter if I use ' or ". Some php guides say I should use include ('beginning.txt') some say I should use include ("beginning.txt").