Is there any way to save pages with the extension ".php" instead of the default ".html", or can this only be done manually after exporting?

Currently this has to be done after export.

Yes PHP is easy to include for example the menu, so when the menu changes need not be processed all pages.
Or is there another way into the HTML editor to construct a library?

If you are asking about the HTML Editor, the programme, it has a library function. It does not work like php includes, though, but you can create some page element once, save it to the library, and then add it to the other pages where you want the same element, with just a click.

Yeah right, but if you went one way makes it nice that you change it changed in one place and then have the choice to let the other pages.
My idea is like PHP or may not in RSD, also placing construct a library in RSD so addresses and menus can be changed in one place and you should not do all the pages manually.

sory for my english below in dutch,

Ja klopt, maar als je dan een wijzeging maakt is het fijne dat je het op één plaats wijzigd en de keuze hebt het dan ook op de andere pagina's te laten wijzigen.
Mijn idee is als PHP dan niet kan in RSD, ook een bibiotheek in RSD te plaatsen, zodat adressen en menu's op één plaats gewijzigd kunnen worden en je niet alle pagina's handmatig moet doen.

You can always setup your server to parse php inside a html page.

With or without php extension for the main file(s), the actual php files that are to be included will have to be created separately after the main files have been exported. The way the library feature is made now, there is no need for it if you include php files, the 'included' folder is enough. But of course, it would be nice if the Editor, and also RSD could handle this inside the programme.

So if I am understanding what I read php is not integrated into RSD or the HTML editor? I should be learning to do footers for example as a library item and add to a page in this manner. One of the many things I do with an include is addresses. That way if a business moves I can update the required info in one place. For very large sites this is a major time saving thing.

The HTML editor has some support for PHP. You can write code and there's a tab full of php tags but you'll need to publish your page to an online server or to a local server setup on your pc to preview.

When I am on Windows using HTML Editor, I'm usually working in PHP. I've found a way to use PHP with RSD that works well for me.

1. I set up the server to parse php within HTML.
2. Wherever I need PHP code, I add an HTML Element with an include statement.
3. In the exported project, I create a folder called includes and put all the files that will be included throughout the site in that folder.

It works very well for me.