So a while ago I created a 1kB PHP forum based on the original.
Since then I wanted to see if I could actually make a useful discussion area by adding a couple more kilobytes of code to the project. I also wanted to move to SQLite so that I could use this system on lowendboxes without needing to setup a full RDBMS. The main PHP file is ~4kB, the theme is another ~6kB, and the rest is the twitter bootstrap framework.
It may be ugly, but it uses UTF-8 encoding, prepared statements, and is E_STRICT.