Just to confirm, is using:
$_SERVER[\"DOCUMENT_ROOT\"]
the same as using: /
in HTML.
Eg. If current document is:
Just / refers to the root of your website from the public html folder. DOCUMENT_ROOT refers to the local path to the folder on the server that contains your website.
For example, I have EasyPHP setup on a machine...
$_SERVER["DOCUMENT_ROOT"] gives me file:///C:/Program%20Files%20(x86)/EasyPHP-5.3.9/www but any file I link to with just / will be relative to my www folder.
If you want to give the absolute path to a file on your server (from the server's root) you can use DOCUMENT_ROOT. if you want to give the absolute path to a file from your website's root, use just /.