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().