Sometimes, you have x1, y1, x2, y2 parameters, but not the center, width, and height. Use this to draw a filled ellipse from your x1, y1, x2, and y2:
ImageFilledEllipse(
$im,
($x1 + round(($x2 - $x1) / 2)),
($y1 + round(($y2 - $y1) / 2)),
($x2 - $x1),
($y2 - $y1),
$color);