[eluser]BrianDHall[/eluser]
Optimization is all about finding the weak link or the bottle neck, and considering both performance and reliability without sacrificing core functionality.
By necessity this means it is utterly dependent on your application, there is no real general advice other than using updated software. Everything after that is dependent on your needs.
For instance, is your application database intensive? If so, you'll probably want a stripped down machine that does nothing but operate your database. If your database is only used for sessions and the like then this could very well slow down your applications.
Do you have lots of static resources being served, like images and videos? Then perhaps they need their own 'content delivery' servers with feature-stripped static file servers instead of Apache that do little more than receive and deliver file requests. If this is not a problem then you might be slowing down your application by utilizing them.
Is your application memory or processor intensive? Why? If you are doing image processing you probably need to use a faster image processor like ImageMagik, which perhaps should be handed off to a "heavy processing" server that will just gently return the finished results when its done and won't bog down the rest of your site. If you do heavy database work then you need to look at stored routines to prevent lots of data exchange between the DB and PHP.
Is portability between OS an issue? If not and you do heavy processing tasks, maybe you need to write a program in a lower-level higher performance language to speed processing - like C, etc.
Trying to get the best of anything requires a definition of what you think 'best' is and what you are looking to get out of it. Take a hotel for example - what's better, Motel 6 or the Waldorf? If you are holding a delegation of world leaders, the Waldorf - if you are going cross-country on a tight budget, Motel 6.
