DOCUMENT_ROOT is a predefined PHP variable, it is part of the SERVER collection. more informatin can be found here:
See this post for adding the Document Root variabel when not present in IIS:
boaz at babylon dot com
You can add $_SERVER["DOCUMENT_ROOT"] to IIS by editing the Environment Variables of your Windows server (was tested on WinXP SP2).
Right click on My Computer >> Properties >> Advanced.
In the System variables click on 'New' and Type in the name field 'DOCUMENT_ROOT' and in the value field the path to your IIS document root folder.
Don't forget to restart your Windows (IIS restart won't load the new settings).