Note: I posted this originally to stackoverflow.com, had it closed, and was told to post it on programmers.stackexchange.com. So I posted it on programmers.stackexhange.com, had it cloased, and was told to post it on webmaster.stackexchange.com. Here's my last attempt to get some help and direction on this nerve-racking problem.
I've been pulling my hair out for weeks trying to decide which way to go.
One of my LAMP sites is several years old and my users have requested a forum. My site is custom-built by myself, and the forum would have to integrate fully with my site so that it actually looks, feels, and behaves like my site. For example, I have a site-wide messaging system on my site that alerts users whenever they receive a new message no matter where on my site they are on, so this feature would have to be extended to the Forum. I would also like to be able to limit access to certain sub-forums based on criteria from my site as opposed to criteria from the forum.
I don't need a full-featured forum as it will not be the main focus of my site, just a feature of it.
The question is, do I use an existing forum platform and integrate it with my site or do I just build one from scratch?
The pros/cons are relatively obvious, but I can't decide which way to go and it's crunch time.
The pros of using an existing open-source platform is that they are tried and tested, secure, have a lot of built-in features, community support, and would save me a lot of time.
The cons are that they seem to severely limit how tightly and well I can integrate it with my site and my site's features, not to mention that most are overkill for what I need (bloatware). All the forum packages I have looked at (and I've looked at MANY), are all front-end solutions that will behave like an external add-on to my rather than an integral part of it. I have not found any fully-customizable "backend" forum solutions. This means that I will need to customize the forum to fit in with my site, but if that means altering core files, it will make the forum difficult to upgrade.
Building the forum from scratch means I need to re-invent the wheel with all the potential problems that might raise, but on the other hand, I can program the forum so that it does everything exactly the way I need it to, making it an integral part of my site rather than an external add-on to it.
So, what do you recommend and why?