Just to confirm, is using:
$_SERVER[\"DOCUMENT_ROOT\"]
the same as using: /
in HTML.
Eg. If current document is:
Yes, on the server side $_SERVER['DOCUMENT_ROOT'] is equivalent to / on the client side.
For example: the value of "{$_SERVER['DOCUMENT_ROOT']}/images/thumbnail.png" will be the string /var/www/html/images/thumbnail.png on a server where it's local file at that path can be reached from the client side at the url http://example.com/images/thumbnail.png
No, in other words the value of $_SERVER['DOCUMENT_ROOT'] is not / rather it is the server's local path to what the server shows the client at example.com/
note: $_SERVER['DOCUMENT_ROOT'] does not include a trailing /