I think we certainly have had PHP built infrastructure that scales, but surely you can see why an order matching system should not be written in PHP.
First of all, by doing it that way, the order matching system was coupled to the website, so now it makes perfect sense why the BitCoin price crashed after the DDoS attacks on MtGox. Because taking the website down meant taking the order matching down with it. No more trading.
From what I am seeing, this is not a case of PHP being evil (although, would you really run mission critical systems with PHP? The execution model doesn't make sense in that world and if you think a set_time_limit(0) on a PHP script is the same as an actual daemon written in a robust language meant for that execution model, then I think we are in extreme disagreement).
For me this is a case of a guy who's confidence was ahead of the reality. I'm sure in his mind a pacemaker running on PHP code is perfectly fine, and perhaps it might actually work for a while but that's just it, it will fail eventually (its a square peg in a round hole after all) and when it does it will be bad. We don't craft critical systems thinking of the best case scenario, we do it thinking of the worst and for my money, whatever happened at MtGox, its the worst case scenario.
PHP in Facebook, yes, NYSE? FUCK NO.
Also, for the GP, PHP's shared-nothing architecture means scaling horizontally is actually pretty easy all things considered. I just wouldn't write a bitcoin exchange in anything that doesn't have static typing: PHPs gotchas could quite literally cause a massive loss of money.
Languages do not scale, application designs do. You can even make BASIC scale if you design your app for that purpose.
Besides, nothing impressive about PHP running the "largest Bitcoin exchange". Bitcoin, in the grand scheme of things, is an insignificant niche, not even 1% of the world population has anything to do with it at the moment.
PHP runs much bigger stuff, like Yahoo! and of course Facebook (sure, Facebook now has custom PHP JITs et al, but used to run on just PHP back in the day when it was already huge).