It seems like a giant can of worms to introduce mod_php. Do you allow sessions? Replication? How do you handle caching? These are non-trivial problems at CDN-level loads.
Offering 100% availability is crazy difficult once you add dynamic content into the mix. If you could do it, I bet you could make a million bucks.
Availabity with dynamic content is easy (ok not easy, but it's a fairly routine straightforward process). The CAP theorem says you get 2 out of the 3 of consistency, availability, partition tolerance. Just getting availability and partition tolerance by themselves is just a matter of good engineering, nothing revolutionary.
Offering 100% availability is crazy difficult once you add dynamic content into the mix. If you could do it, I bet you could make a million bucks.