<a href="<?php echo $_SERVER['DOCUMENT_ROOT'].'/hello.html'; ?>">go with php</a> <br /> <a href="/hello.html">go to with html</a>
Try this yourself and find that they are not exactly the same.
$_SERVER['DOCUMENT_ROOT'] renders an actual file path (on my computer running as it's own server,
/ renders the root of the server url, in my case,
localhost/hello.html are in fact the same file