Migrating to phpBB3, I want to extend it as I used to do with phpBB2. That said, you can assume that I am a little bit familiar with the code base of both and that I "speak" PHP fairly fluent to get the stuff done.
Now, the built-in BB codes are implemented through
message_parser.php. That also includes support for the simple BB codes that can be created in the ACP.
However, it appears that there is no hook and no other support to easily create advanced custom BB codes. In my case simple substitution of strings will not be sufficient for several reasons, with the most important one being that I need to make DB queries when evaluating the contents of the BB codes.
To give you one example:
... should yield the full title of the linked topic or a red text saying that the topic does not exist, but:
... should link
text to the topic identified by the ID.