Yeah...just read all the php files in the YaBB SE package and disect the mods that are currently available. I'm just a beginner, but I learned a bit by reading the source code and how they all relate and connect to each other. When you find a "function" that you want to know the definition of, do a search on
www.php.net to find out what it's supposed to do and/or
www.mysql.com to learn how to connect/write to the database.
Take it from an eighth-grade dropout. I learn like Abe Lincoln....read, read, read, write.