[Konstantinos]

Why don't you use ZCMD with sscanf and you use strcmp which is slower. It's also easier!

pawn Код:

CMD:hi(playerid, params[])
{
    if(isnull(params)) return SendClientMessage(playerid, -1, "/Hi <message>");
    new
        str[ 128 ]
    ;
    format(str, sizeof(str), params);
    SendClientMessage(playerid, 0xFFFFFFFF, str);
    return 1;
}

Also, isnull can be used for this situation and it's better than sscanf. Any other parameter, just use sscanf as you did.