ok, so far, your spry dataset should contain all of the xml rows exported by the menu.php.
you should be able to pass the url parameter when setting the dataset.
your dataset line should look something like this...
var ds1 = new Spry.Data.XMLDataSet("menu.php", "items/item");
you want to pass in the url parameter here, can't remember the syntax, but something like...
var ds1 = new Spry.Data.XMLDataSet("menu.php?detailid=<?php $_GET['detailid'] ?>", "items/item");
or something like that.
this should (hopefully) filter the xml
sorry, I'm on UK time here, if this doesn't work, post back and i'll take a look tomorrow