[elliottcarlson]

If you are able to build everything with a high-scalability mindset from the get-go there is no reason why PHP can't be used to such a capacity. There are naturally hurdles that you will overcome as well - but realistically there will be scalability hurdles in any dev environment you choose to scale to those kinds of numbers.

Remember to cache, load balance and minimize disk i/o and you already have a good head start.

On a side note, Foursquare is using Scala with the Lift framework, not Vala. And from what I understand, Harry wasn't the original developer when it came to the PHP version of Foursquare (and if I understand correctly, the original developer wasn't even an engineer). When the time came to move on to something bigger and better, Harry rewrote it using Scala because of various personal preferences (Java background, preference to compiled and typed code, etc). So the decision to move from PHP because of performance issues may be more related to poor quality of code (as stated in this slide deck: https://docs.google.com/present/view?id=dcbpz3ck_25czcns2c2&...) than the actual platform.