No, I haven't noticed the last edit
Thanks for your suggestions,
Dwarfman, it really helps!
Well, no, the "key" won't be in the source code, and I cannot let anyone build a "full online" client for the following reason: anyone can modify the game, add more damage to weapons, more HP, weaker monsters, increase scoring, etc...
For the "online score" version, the players should play with the same version.
I will have a look at the hash library - or I will use MD5 (it's easy to find implementation for C++ and PHP) - and at the sfml-network header.
For the game-logic-related test, I prefer to do this manually, if some scores are strange.
At the moment, I'm implementing threads to prevent the game to "freeze" while waiting for server response.
I've noticed my compiler does not support C++11 thread, so I've just upgraded it.