I have run into this problem so many times: I know my email address, but I do not remember the userid that I used at signup time. phpBB will not send you the userid, and it will not allow you to register a new userid.
There are many bulletin boards that use phpBB. When you register, you have to pick a userid that has not already been used. So, even though I can use my real, globally unique, email address to register, I often have to pick different user names at each BBS. It is a bother to diligently write down and record all the user names I have used at all the various BBS systems for various product support forums.
The email address should BE the ID used to log in. This is so simple. You should be able to specify a Name that gets displayed, but that is all it is, just a name, and it would not have to be unique. Or even if it was, you should be able to log in using your email address and your password. The system already assures that email address is unique, so if you have the password that should be enough to log in. Why complicate things by making you remember and track an additional quantity?
I have complained on various BBS systems about this, but that is the wrong place. Those people are using phpBB and are not bothered with the theory behind a security system. the developers of phpBB need to form the software with the right possibilities in the first place.
As I registered for this forum as well, I notice that this version of the software, which I assume is the latest, still has the same problem: you are forced to enter a unique username and you must remember it in order to log in.
One simple solution I have used in other software is to use my email address as my user name, but that is not allowed. Apparently there are restrictions on what the username can be. But that is basically a hack.
The simplest solution is just ALLOW one to log in using email address and password. Using the email you can find the username, and if the password matches, then there is no reason not to log the person in.
PLEASE PLEASE PLEASE make this simple change, make your already good software much easier to use, and in a single stroke fix thousands of BBS at the same time.