using Apache variables as a parameter to a PHP script

I have a image tag that is generated by a php script

<IMG
src="http://lww.com/pphlogger/showhits.php?id=user&st=img&mpdl=/cleansers.html">

The last parameter (mpdl) is defined as the page I am getting the
information for (it is a hit counter).

in most cases it is the current page, so I would like to use
$DOCUMENT_URI instead of manually typing in the page on every link.

I have found I can not just refrence the variable.

Thank you for any help,

Steve