Description:
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the imagefilltoborder call will flood fill upwards only. 
it's pretty easy to see : http://
ldab.arts.kuleuven.ac.be/gdtest.php

I ran it on 3 other macs, clean installs with the PHP 
module from www.entropy.ch, which has very good fame 
among OSX people.

the weird part is that linux code works fine... 

I downloaded sources from 4.3.6 up to 4.3.9, and 
compared the gdImageFillToBorder calls. They are 
identical (even the original GD from Boutell has the 
same code) so I really don't know what could be wrong.

cheers

Reproduce code:
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header("Content-type: image/png");
// create a blank image
$image = imagecreate(400, 300);

// fill the background color
$bg = imagecolorallocate($image, 0, 0, 0);
// choose a color for the ellipse
$col_ellipse = imagecolorallocate($image, 255, 255, 255);

// draw the ellipse
imageellipse($image, 200, 150, 300, 200, $col_ellipse);
imagefilltoborder($image, 200, 150, $col_ellipse,$col_ellipse);

// output the picture
header("Content-type: image/png");
imagejpeg($image);

Expected result:
----------------
a fully white ellipse on a black square

Actual result:
--------------
on a black square, an ellipse with white border, upper 
half filled white