new PlayerCount[MAX_PLAYERS]; //variable to hold the players countdown value
new PlayerCountTimer; //timer id for countdown timer
forward PlayerCountDown(); 

public OnGameModeInit()
{
    PlayerCountTimer = SetTimer( "PlayerCountDown" , 1000 , true ); //sets one second loop, you could also use an existing timer loop
    return 1;
}

public OnPlayerConnect( playerid )
{
    PlayerCount[playerid] = 0;
    return 1;
}
public PlayerCountDown() //this function is called every second
{
    new
        string[6]
    ;
    for( new i; i < MAX_PLAYERS; i++ ) 
    {
        if( IsPlayerConnected(i) )
        {
            if( PlayerCount[i] > 0 )
            {
                PlayerCount[i]--;
                if( PlayerCount[i] == 0 )
                {
                    //put code in here, depends on the function I guess
                }
                format( string , sizeof(string) , "~r~%d" , PlayerCount[i] );
                GameTextForPlayer( playerid , string , 1000 , 3 );
            }
        }
    }
    return 1;
}