Why do PHP pages want to open as files?

Question

PHP

After migrating a site from http to https, sometimes (randomly) PHP pages try to open as files.

Screenshot:


When I try to open it in a text editor, nothing special happens, it opens an empty file. The site has been spinning on a regular hosting for a long time and there has never been such a problem. What is it at all and how can it be fixed?

Browser - Firefox, if that matters.

Answer 1

I used to have such problems.
Add the following lines to .htaccess at the site root (or to apache.conf or httpd.conf in the /etc/apache/ folder):
AddHandler application/x-httpd-php .php