Recent Comments feature

You have saved it as .phps
Change it’s extension to .php
If you get a “Headers already sent” error, https://faq.wordpress.net/view.php?cat=7
(I thought I answered this yesterday … must be going mad ?? )

I uploaded it as recent-commenters-commented-on.php but still there’s the exact same error page.

Did you use something like Word to save it ?
If yes, use Wordpad.
Wordprocessors screw up code.
It appears that although the code is php, it’s being read as text, hence this answer..

Open up the phps page in your browser. Open up a text editor like Notepad or Textpad. Copy all the code in your browser window and paste it into your text editor. Save it as “recent-commenters-commented-on.php” use that as your plugin file.