Function PlayerTextDrawSetOutline set the outline of a player-textdraw. The outline colour cannot be changed unless PlayerTextDrawBackgroundColor is used.
Function PlayerTextDrawSetOutline was added in 0.3e and will not work in earlier versions!

(playerid, PlayerText:text, size)
int playerid The ID of the player whose player-textdraw to set the outline of.
int text The ID of the player-textdraw to set the outline of.
int size The thickness of the outline.

Return Values:
This function does not return any specific values.

new PlayerText:MyTextDraw[MAX_PLAYERS];

MyTextDraw = CreatePlayerTextDraw(playerid, 100.0, 33.0,"Example TextDraw");
PlayerTextDrawSetOutline(playerid, MyTextDraw, 1);

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