For others coming here, in case you a wondering after the many posts from MulayamSinghYadav: The file in post #20 seems to work great in MyBB 1.8.15.
@MulayamSinghYadav: You keep asking for help. Did you find a solution? If you still have a problem, maybe it will be easier to help you if you told us what you have tried and what is not working as expected.
I simply downloaded the file in post #20. Renamed it to feed.php. Uploaded it to the root of my forum, and I can get the feed just fine using an address like
https://example.com/forum/feed.php