imagearc() draws a partial
ellipse centered at cx, cy (top left is 0, 0) in the
image represented by image.
W and h specifies the ellipse's width and
height respectively while the start and end points are
specified in degrees indicated by the
s and e
arguments. 0° is located at the three-o'clock position, and
the arc is drawn counter-clockwise.
See also imageellipse(),
imagefilledellipse(), and
imagefilledarc().