I haven't the faintest. I looked for mysql.so, and it's located in /usr/lib/php/extensions, like it's apparently supposed to be. I don't have libmysqlclient.so.12 anywhere on my system, though (like I said, I don't have MySQL installed).
I'm not really looking to solve the problem as much as I am for a way to make php not complain so much. Maybe a "quiet" option or something?