Why not use JavaScript in place of PHP?
The game engine is written entirely in JavaScript. PHP performs data conversion tasks for which its standard library is much more suited (with all those gd, zlib, zip, etc.). Currently, the amount of PHP code is about half that of the JavaScript code and its share will go down as the engine evolves.
Eventually, all PHP code may be rewritten into JavaScript. However, this is a low priority task because it won’t improve the game experience for the end user at all. It will eliminate certain duplicate code but efforts needed to carry such a refactoring are unproportional to that code’s amount.
That’s all for now. Have other unanswered questions? Ask away!