If your exchange involves serving web pages http://bitcoindevkit.com is your library, However if you want to server pages AND do realtime proccessing your going to want to do a C++ and PHP method. PHP will serve pages, and C++ will do the proccessing however I don't have a C++ library for the Bitcoin Dev Kit project as of yet just the PHP so if your project can do with out the realtime and huge loads (which for a school project shouldn't be an issue) then you can create a PHP only exchange (which isn't recommended for anything serious like MTGox, you should defiantly use PHP and C++ like stated before.)
Ahem, you don't want PHP, C++, or {insert whatever language here}
You want something that serves a specific purpose, ease of maintenance, performance, etc.
Why not Ruby+Redis ? Why not Node.js+Mongodb ? Why not Erlang+Cassandra ? Why not PHP+MySQL ?
It's not the tool that matters, it's the skill with which you use it.
Thanks again davout for your lovely insight.
