Function SendClientMessageToAll displays a message in chat to all players. This is a multi-player equivalent of SendClientMessage.
Avoid using format specifiers in your messages without formatting the string that is sent. It will result in crashes otherwise.

(color, message[])
int color The color of the message (0xRRGGBBAA Hex format).
string message The message to show (max 144 characters).

Return Values:
This function always returns true (1).

public OnPlayerCommandText(playerid, cmdtext[])
    if (strcmp(cmdtext, "/helloworld", true) == 0)
        // Send a message to everyone.
        SendClientMessageToAll(-1, "Hello!");
        return 1;
    return 0;

Related Functions
The following functions may be useful, as they are related to this function in one way or another.

Related Callbacks
The following callbacks might be useful as well, as they are related to this callback in one way or another.
  • OnPlayerText: Called when a player sends a message via the chat.