[Dayrion]

Well, if you know how to use sscanf, I don't understand why you don't know how can we use isnull.
Anyway, let's take a simple example (including zcmd):

PHP код:

CMD:adminchat(playerid, params[])
{
    // check if the player is admin or what ever
    if(isnull(params))
        return SendClientMessage(playerid, -1, "/a [text]");
    // do stuff
}