From phpBB Development Wiki
Rather than building our own DBAL we could use a 3rd party library for this. This would make even more sense if we choose to use an ORM.
Could be based on our work in phpBB3. However a major rewrite would have to take place to make the code comply with phpBB4 programming concepts.
A good option appears to be Doctrine 2 (documentation). The ORM part would certainly come in handy. Currently it has the following drivers:
So we would have to write some more drivers to get the full database support we have in phpBB3. But since we would need to make major changes to those anyways we can as well turn them into Doctrine DBAL drivers and contribute something to Doctrine that would be useful for all other Doctrine users.