Often if you have a really loader server-environment you can do so much more to reduce the overhead and overall load by just using some sample optimizations tips in PHP, tweaking Apache, installing cache-servers (memcache, turkmm etc.) or playing with the indexes in your mysql database. People that don’t mind paying thousands of dollars more on hardware can skip the tips provided below 
The last couple of days I’ve been playing with memcached which allows to use the server-ram in a very robust way as though it’s another sql-db, thus many times faster! You can easily achieve great results by dumping the most common and accessed data onto the RAM instead of using whatever database-scheme. Even heap-tables with mysql include some overhead so direct-memory caching is way faster.
There is the link to a very extensive tutorial on server and script-optimization: http://phplens.com/lens/php-book/optimizing-debugging-php.php.