Optimisation is a tricky topic and something you should be very careful about when you are taking advice - even if it is my advice! Donald Knuth said that "premature optimization is the root of all evil", and this is quite true. Having said that, there are two books that cover optimisation very well indeed, one for PHP and one for MySQL, so if you really want to take your scalability even further, give them a try.
For MySQL, go and buy a copy of Paul DuBois' book, "MySQL". This is also listed under "Further reading" for the Databases chapter of this book, which means it's doubly recommended - can't be bad!
For PHP, consider reading George Schlossnagle's book, "Advanced PHP". It is pitched at quite a high level, and some parts of the book drag on longer than they should, but it is the only book currently available that deals exclusively to making PHP work in highly scalable environments. Plus George is an active and well-respected PHP developer - what he says is usually worth listening to.
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