in the while the code may will change, so something here will be added or updated
You should (but could be not) start the integration, with corresponding and unique email and usernames pairs (as it has been until now).
phpBB need to be setup on ACP to not share same email for different usernames, and DO NOT allow usernames changes (but since the integration works by email now, could be allowed to change usernames, but this aspect will be covered after, since with changing these rules, concepts of an integration become more hard, that is may clear for those that knows advanced CMS logic and web programming, and considering that this is a WP plugin, that so, only run in WP side).
Possible logins integrations:
If user's profile email update allowed in phpBB, then the user's login and registration is possible to be allowed ONLY in phpBB.
If user's email update disabled into phpBB user's profile, any other combination between wp and phpBB is possible, included (but it is discouraged since 2.4.0>) leaving users register on both wp and phpBB.
It is possible to switch to one or other way to integrate, any moment
2.4.0, is not anymore by username, but by email. And an email can be updated to something else.
SO it is important to fix in mind that for this reason, it is mandatory to choose, and users need to be forced to update their email or register in WordPress OR phpBB (and in this case also the login need to be allowed ONLY in phpBB), not on both.
Usernames scenario example:
phpBB usernames (if they exists, but it remain suggested to setup phpBB ACP to not allow phpBB to accept any usernames chars) like:
will be added in wordpress as:
because this is the default wordpress result after the username has been "cleaned".
this username, added in wordpress if not exist, can be added in wordpress because the user come to visit wordpress as logged in phpBB, or due to a login done in wordpress side. The user can login in wordpress using $£ALE'-\=0, but will be created as ALE-0, so first login will fail if done as phpBB username, the message will display that the username do not exist and invite to try by email. The user will have to use ALE-0 (the wp username), if logging in wp side by username and not by email, after it has been created in wordpress.
The user's Display name option of the user at this time, will display the complete phpBB username not sanitized, so $£ALE'-\=0.
Anyway, note that wordpress if the login done by user_login -> $£ALE'-\=0, so this user will have to login in wordpress side, using his email and NOT his phpBB username, or will have to use ALE-0 (the wp username), if logging in in wp side by username and not by email, after it has been added.
On WordPress MUMS, any phpBB username containing characters that aren't into the range [0-9A-Za-z]/' (lowercase Uppercase a-z, and 0-9) will not be added into wordpress-
User ID1 in WordPress and user ID2 in phpBB (default install admins) are not linked anymore.