I've also ran into this problem. There is a 'contants.php' located in the 'WebServerExtensions\www\mapviewerphp\' folder, so I am guessing you make a copy of that, drop it in the sample folder then fix up the paths? however I havent managed to get it to work.
I have created the Invoke URL task, then viewed the "Edit Markup" page, but nothing works beyond that.
Please get back to me if you have shed any light or had any breakthroughs, thanks, Joey.