[Patrick]

I think this is what you want, explanation inside.

pawn Код:

public OnPlayerText(playerid, text[])
{
    new 
        string[ 128 ], PlayerName[ MAX_PLAYER_NAME ]; // variable, so we wouldn't get undefined ....

    GetPlayerName(playerid, PlayerName, MAX_PLAYER_NAME); //this function gets the name of the player.

    format(string, sizeof(string), "%s: %s", PlayerName, text);
    SendNearByMessage(playerid, -1, 20.0, string); // will send the message who's in range of 20.0
    return false; //return false, because if it's true, the message will be sent twice.
}

stock SendNearByMessage(playerid, color, Float:radius, const message[])
{
    new 
        Float:Position[ 3 ], i = 0
    ;

    GetPlayerPos( playerid, Position[ 0 ], Position[ 1 ], Position[ 2 ]);
    while(i < MAX_PLAYERS)
    {
        if(!IsPlayerConnected(playerid)) continue;

        if(IsPlayerInRangeOfPoint(i, radius, Position[ 0 ], Position[ 1 ], Position[ 2 ]))
        {
            SendClientMessage(i, color, message);
        }
        i++;
    }
}