TL;DR I would use Scala, F# or Rust. If you don't like FP, then Java/C#.
Statically typed: The reason I would avoid Php, and most other dynamically typed languages, is to avoid bugs. Dynamically typed langs are easy to learn and code, but a statically typed lang like Java / C# will avoid a certain kind of bugs, and make refactoring easier.
Automatic memory management: I would also avoid langs where you manage memory, like C/C++, mostly because I fear bugs. Remember Heartbleed? That category of bugs can only happen when the programmer manages memory. Rust/Go are better.
Functional: I know that FP is considered hard, but it is a way to avoid bugs, especially if you are running a multithreaded app where the threads communicate with each other (like trading). FP's most important feature is that you work with immutable data structures. Functional langs are Haskell, F#, Scala and Erlang.
It's what you do with it that counts :) PHP isn't terrible, but it's not the right tool for an exchange back-end. I know some use C/C++.. I suspect most do.. for speed/efficiency and code quality. – George – 2014-11-20T06:44:21.807
1
@George, take a look at http://meta.bitcoin.stackexchange.com/questions/633/please-post-answers-as-answers-not-as-comments?cb=1.
– morsecoder – 2014-11-20T16:53:42.673fair enough. I didn't think it quite warranted being a full answer as I don't actually know what most exchanges use so it was more of a comment than an answer. – George – 2014-11-20T17:47:51.077
1@StephenM347 I don't think George's reply could serve as an answer, not at least until he decides to get into more detail and justify his arguments. – George Kimionis – 2014-11-20T19:58:28.140
Hey new_to_bitcoin, it seemed to me that your underlying question was rather to ask a best-practice recommendation and reasoning, than just an iteration of the status quo. I've thusly edited your question. If you feel that it's not what you wanted to ask anymore, feel free to rollback or further modify. For a short intro how our site works, check out [tour] and [ask]. – Murch – 2014-11-23T13:28:10.167