Function GameTextForAll shows 'game text' (on-screen text) for a certain length of time for all players.

(string[], time, style)
string string The text to be displayed.
int time The duration of the text being shown in milliseconds.
int style The style of text to be displayed.

Return Values:
This function always returns 1.

public OnPlayerDeath(playerid, killerid, reason)
    // This example shows a large, white text saying "[playerName] has
    // passed away" on everyone's screen, after a player has died or
    // has been killed. It shows in text-type 3, for 5 seconds (5000 ms)
    new name[ 24 ], string[ 64 ];
    GetPlayerName( playerid, name, 24 );
    // Format the passed-away message properly, and show it to everyone:
    format( string, sizeof(string), "~w~%s has passed away", name );
    GameTextForAll( string, 5000, 3 );
    return 1;

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