Description:
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imagearc() draws broken arc, when using negative value as s parameter.
It could be GDlib bug, at least I found a couple of words in GD man: http://www.boutell.com/gd/manual2.0.23.html about unspecified bugs in gdImageArc().
Also, there can be simple workaround: just don't accept negative values, reduce them to zero.

Reproduce code:
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// result image is here: http://tony2001.phpclub.net/temp/imagearc.png
//
// create a 200*200 image
$img = imagecreate(200, 200);

// allocate some colors
$white = imagecolorallocate($img, 255, 255, 255);
$black = imagecolorallocate($img, 0, 50, 200);
   
// draw a white circle 
imagearc($img, 100, 100, 150, 150, -1, 200, $black);

// output image in the browser
header("Content-type: image/png");
imagepng($img);
   
// free memory
imagedestroy($img);


Expected result:
----------------
a flat arc

Actual result:
--------------
a broken arc with some crosslines