Your web host would be able to tell you what the value of $_SERVER['DOCUMENT_ROOT'] should be. Alternatively, if your website supports PHP, you can create a file with the following code in it and upload it to your web server. Call it something like phpinfo.php, or something else just as long as the file extension is .php.
Code:
<?php
phpinfo();
?>
Upload it to your root directory, then run the page on your web server. For example, if you named the page phpinfo.php, you would type the following URL in your browser:
http://serverbox.hostingsite.com/phpinfo.php
Within that page, there should be an entry for DOCUMENT_ROOT. That's what you need.
One last thing: Once you have obtained this information, don't leave that file up on the server, as it contains sensitive information that you wouldn't want an unauthorized person to be able to access.