I guess you are retrieving the function in your plugin in such way (or similar):
PHP Code:
OrpheuGetFunctionFromObject(g_pGameRules, "CheckWinConditions", "CHalfLifeMultiplay");
I think the last parameter has to be "CGameRules" instead of "CHalfLifeMultiplay". Not sure though, I don't know how Orpheu deals with class inheritance.
Show us how you retrieve the function in your code.