A few weekends ago I upgraded php to the latest version. I also broke my largest remaining phpBB2 board.
My first (and largest and still active) phpBB2 board was based on the 2.0.4 codebase. Over the years I implemented some (but not all) of the suggested upgrades, because as soon as I got the code I started changing it. That made applying updates a bit more challenging. Anything that was related to security (such as the hole in the search code) was patched, but other items were not necessarily patched.
One of the bits of code that was added later along the line was to check the $_POST array for values and restore everything back into $HTTP_POST_VARS. I never bothered to do that, and until last month everything still worked fine. With the latest version of php, they finally stopped supporting those arrays altogether so I had to update my code accordingly.
The board is functional again, but the writing is on the wall. phpBB2 is probably (finally) on the way out.