On the page https://www.phpbb.com/extensions/ the links to Extension development tutorials (one at the menu on the right, plus 2 at the left column) are wrong.

https://area51.phpbb.com/docs/dev/31x/extensions/

https://area51.phpbb.com/docs/dev/32x/extensions/

should be (missing dots in 31x/32x parts)

https://area51.phpbb.com/docs/dev/3.1.x/extensions/

https://area51.phpbb.com/docs/dev/3.2.x/extensions/