Can you believe people have even made YouTube videos to help people find
the version number of a given Phpbb installation?

https://www.google.com/search?q=find+phpbb+version

Then there is also Phpbb Version Grabber - Exploit Article
http://www.governmentsecurity.org/forum/topic/18305-phpbb-version-grabber/

Let's face it, there will always be a way do dig it out or infer it.

Therefore, at least for default installations, please instead just
finally do what Mediawiki etc. do, and put it in the HTML source of each
page:

<meta name="generator" content="MediaWiki 1.24wmf17" />

Why would one ever need to know the version number for a site they
didn't administer? Well that way they could file more accurate bug
reports here.