I don't know php but we've been using MySql for close to a year and I have to say it is the way to go. The MySql plugin is great (has a small memory leak but nothing major), MySql is free, and it's actually easy to setup and use!

Not adding much to the discussion here except to say that if you can, take the time to play with MySql and use it instead of a flatfile system whenever you can... whatever time you spend learning it will pay off in droves in the future in terms of lowered development times and increased performance.