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PHP код:

new last[MAX_PLAYERS]; //global
COMMAND:hello(playerid,params[])
{
    if(last[playerid] < gettime())
    {
        last[playerid] = gettime()+5;
        //your "hello"-command
        return 1;
    }
    else
    {
        new string[145];
        format(string,sizeof(string),"You have to wait %i seconds.",last[playerid]-gettime());
        SendClientMessage(playerid,-1,string);
    }
    return 1;
} 


Here is a second example with gettime(). Now you have two options how you want to realize this command.

@Sascha:
SendClientMessage can maximal sends 145 letters.