So the Ascraeus blog post
states that PHP 5.3.3 or better will be needed for 3.1.x :
Please note that phpBB 3.1 has some requirements that differ from phpBB 3.0:
PHP version 5.3.3 or greater
PHP’s json extension
Only PostgreSQL versions 8.3 and above are supported
However, the very first post in the follow-up forum topic
The next major release of phpBB, phpBB 3.1 "Ascraeus", will require PHP 5.3.2 or newer.
It would be nice to know which it is : 5.3.3 or 5.3.2 . Any insight much appreciated!