you can force site sculptor to always create php files by editing one of the source files.
you will need to first locate the users configuration folder, see this tutorial for details on locating the configuration folder:
in the configuraton folder go to:
and open the shared.js file. change line 844:
return MM.SiteSculptorPageExt ? MM.SiteSculptorPageExt : '.html'; //".php";
return MM.SiteSculptorPageExt ? '.php' : '.php'; //".php";
make sure to close DW and delete the cache file from the configuration folder then restart DW before creating the new site.