I scrolled through this thread pretty quickly so perhaps I overlooked something.
If I did not, I cannot believe no one asked the obvious question:
Why, for whatever you value most, would you want to create a forum application? It has been done to death. Yes, it is fairly easy, yes it has been done before with success, even by latecomers. But as learning experience you've already gotten the best out of it, and probably did so before you even posted on this forum. You're not going to learn anything substantial by having some random dude post XSS vulnerabilities on some app that'll never take off. Codewise, all I need to see are some tiny signs in your code to know that you are way behind in the herd. Seriously, your code includes a statement "or die("whatever")". This was not acceptable 10 years ago, now it's nothing less of a sign of incompetence. I'm sure some popular OSS apps still use it after all this time, all the more reason to stay the fuck away.
With the intention of providing some usable advice, I had a look at your profile and found a topic where you made a small effort to attack Symfony, but gave up because you couldn't figure it out. I get that. Symfony uses way too much magic. I like it for the clean code and unbeatable completeness, but hate it for its (intentional) tendency to rely on "conventions", ie "magic". I can't count the times the DI config of a bundle wasn't picked up because the naming of a file/class was off slightly. I've since mastered the beast, but I can't fault you for hating on it. Still, Symfony2 is currently the best framework for PHP, once you accept it's an ugly bitch, the bitch will work her ass off for you.
But you'd be smart not to commit to any infrastructure layer, be it from Sensio or from your own hand. This post may be of use to you: http://blog.kleijnwe...y-model-part-1/
To the point, you may want to abandon this hobby project to start fresh for the purposes of learning, and think of something more original. I would apologize for being blunt if I did not believe I'm doing you a favor.