php / mysql performance resources

Hi everyone, Can anyone recommend any good books/resources for php and mysql performance management? I'm more interested in the software development side (as opposed to server configuration) of things. I'm looking for anything that would be good to know when working on websites that get millions of hits a day. Also, if anyone knows of any resources/discussions that illustrate the relative performance of joins vs multiple selects I'd love to check it out. Thanks, Guillaume