I have an existing php website with a user system already set up (registration, authentication, lost password, etc.). I've decided that I want to add a message board and integrate it with the existing website: registration on one should register on the other, and similarly with logging in and changing user information.
Is there a good forum that would make such integration easy? It seems that the most common way this is done is simply to abandon the existing user system and use the forum's user system. But this has downsides: you rely on the forum to properly handle security (on the other hand, this could be a benefit) and you are much more tied to this particular forum (more complex to switch forums, forum problems could cause problems with the entire website, more difficult to have forum on a separate server).
What is a good way to approach this?