Clicking on edit button on any wiki page sees a PHP file download to user.
user trying to edit page, not download PHP file.
Deselect options for following:
"Use external editor by default (for experts only, needs special settings on your computer)"
"Use external diff by default (for experts only, needs special settings on your computer)"
I have been searching for the above solution for some time (years) and only came across the solution when the wikimedia server crashed today and was asked to report the error to webchat.freenode.net #wikipedia channel. Thanks to user Prodego for providing the solution, above.
I asked Prodego if there was a way to find out how many users might have the same options selected and causing the same problem. She/he had only seen it five or six times previously; confirmed that it was possible to find out how many, but that it was unlikely to be answered in the IRC, before referring me to WP:VPT which in turn led me here. No one on the channel knew what the edit options were for.
That the options above not be offered, if indeed there are a significant number of editors who, mistakenly or otherwise, have selected them.
4.1 Alternatively, that the options be explained in plain English - happy to help!