You're assuming that network latencies follow the triangle inequality, i.e., that A->C is smaller than A->B + B->C. However, that breaks down because of money. It's possible that the user's ISP is a cheapskate without good peering agreements such that while A->B is fast, they haven't setup the necessary agreements to make A->C fast.

I've personally seen this with a game recently and my own ISP. I can ping their West Coast servers in 20-40ms while pinging their Chicago servers takes 200ms+.

This is actually one huge flaw with the current way IP is done. Because your ISP is closely tied with the last-mile of routing, you have very little control over optimizing those backend layers of routing.

This doesn't explain why you wouldn't just run a thin proxy at the edge and instead want to run a full PHP app. For that, my guess is this makes sense when you don't want to run a full server all the time or if you want it to be rapidly scalable.