the page extension will be determined by the type of page that is opened when the site is created. Once set, it cannot be changed. you will need to delete all of the site pages and re run the site assist wizard. After deleting all of the pages from the site:
1) Make sure the Server language on the testing server tab of the site definition is set to PHP/mySQL
2) Create a new php page and save it to the site.
3) With this page open in Dreamweaver, start the site assist wizard.