I think most people would agree the ability to click on pages and -->Create Page and be able to stipulate the file extension would be a welcome feature.
I wanted to create my login page; which is going to be login.php due to needing php; when i found that it only creates .html so I have to 'remember' to change the file extension AND the LINK in the menu from .html to .php
Thank you for opening this thread! This suggestion has been mentioned a few times and we came up with the idea of renaming pages upon export.
I think that this is the best way to do it, because if we allowed php pages to be created in the app directly, then people would expect to see the result of their PHP code when designing their sites and when previewing, which wouldn't work.