$points = array(
20, // x1, top-left
20, // y1
230, // x2, top-right
20, // y2
230, // x3, bottom-right
230, // y3
20, // x4, bottom-left
230 // y4
);
$image = imagecreatetruecolor(250, 250);
$green = imagecolorallocate($image, 0, 255, 0);
imagefilledpolygon($image, $points, 4, $green );
header('Content-type: image/png');
imagepng($image);
imagedestroy($image);
?>