[AndreT]

The callback must be in quotes and should not contain the round brackets. An example of that would be:

pawn Код:

HTTP(playerid, HTTP_POST, "http://xxx.com/index.php", ?, "onRequestDone");

Also, if I understand you correctly, you've no idea what to feed instead of the ?. I think this should give you an idea:

pawn Код:

HTTP(playerid, HTTP_POST, "http://xxx.com/index.php", "var1=data&var2=data", "onRequestDone");

edit
Ooh, and also, don't forget about the Sockets plugin by BlueG, which allows you to transport data between your gameserver and PHP even better.