I made a form process and broke the code up into different files to stay clean and organized.
Now I am setting up the form for https security.
Do all the files I pull into my page have to be called as https as well?
In which case I can no longer use include(); as it no longer allows relative paths?
Is the solution to use file_get_contents();? Or does only the (master) page itself need to be called as https?
It sounds like you are confusing terminology. HTTPS is the protocol used to request a page from the server using SSL or TLS encryption. That is separate from how you serve the request.
In your PHP source includes are processed server-side. All of the includes will be done before PHP hands the page off to your web server to be returned over the TLS link.
file.php:
In the example above, the user agent (browser) never sees fileA.php or fileB.php. The page request is returned as a single document. You might request it via https://my-server.com/file.php, in which case that is all you need.