i am no spry expert, but i'm not sure why your menu.php can't be used.
i have created datasets using linked 'xml' files, which are really php files generating the xml on the fly.
when adding a spry dataset, choose xml as the data type.
by default, 'files of type' is set to xml, change it to all, and select your php file.
if your menu.php is generating valid xml it should work.