The player's index is passed into chkFlood() function. It's a callback from register_clcmd("say", "chkFlood"). This one registers a client command ( say/say_team, you are aware about them, right? ). So, when a player will say something chkFlood will be called and the player's index is passed into the callback. (You have just to write something in the function header, then you can use it. In this plugin you see "id", but you can write what you want).