The J2EE guy still doesn't get PHP - SitePoint PHP Blog: "Generally people talk about two types of scalability - vertical (adding new processors, disk, memory to your existing 'big box' or buy a 'bigger box') and horizontal (add extra 'boxes' and distribute load between them).

Vertical scalability is easy to implement but generally more expensive long term. There's typically a limit to how much memory, for example, you can add to your existing 'box' and the 'next box up' costs three times the price but only increases capacity by 20%.

Horizontal scalability takes more effort / cunning but can prove extremely successful, as mused in The Secret Source of Google's Power - build a 'super computer' out of dirt cheap parts. For filesystem (and by extension database) replication across multiple systems there's generally a range of mature solutions out there to choose from, many Open Source. Memory replication is another story (now go back to that important point up there)."