www programming

Yes makes sense, thank you. I'll let you know if I encounter issues doing so. Thanks.

All start with config.php that will link the specified phpBB.
then cookies, will be recognized only related to the board with cookie name that will be found into phpBB config db table. You should use different cookies names of course then.
then database, which have of course different db prefixes, if installations are on same db.
I suppose that then you can do it into same domain.
I have +- ever from long time into same browser, several instances on tabs, of different wp and phpbb linked all into same localhost.

Let know if any problem you may encounter

Hello again,
on the same domain, I plan to eventually have several WP and several phpBB installed : how would that work for cookies in phpBB ?

Like, I would have to have
mydomain.com/city1 as WP
mydomain.com/city1/forum as phpBB, synced to /city1
both on database 1

then
mydomain.com/city2 as another WP
mydomain.com/city2/forum as another phpBB, synced to /city2
both on database 2

and so on.

From what I understand, the way you need phpBB cookie to be set up, it is domain wide, and so wouldn't there be conflicts if I have several WP and phpBB on the same domain ? Or just from the cookies having different names it wouldn't be a problem ?