PHP GD2 library inconsistant

Hello there, I am usually quite good at debugging PHP problems, but this one has me stumped. I have a website which generates a image which is a different colour based upon the variables:

$hue = $_GET['hue'];
$size = $_GET['size'];
$kitNo = $_GET['kitNo'];

The code for it is below:

$ar=array(0,0,0,0);
srand($hue);
$onetwoorthree=rand(0,2);
$first=rand(0,100)/100;
$second=1-$first;

$ar[$onetwoorthree]=$first;
$ar[$onetwoorthree+1]=$second;
if ($onetwoorthree+1>2)
$ar[0]=$second;

$im = imagecreatefromjpeg("../images/kits/top_Normal" . "$kitNo" . ".jpg") or die ("Error");

header ("Content-type: image/jpeg");

$height=imagesy($im);
$width=imagesx($im);
$im2=imagecreatetruecolor($width,$height);
for ($x=0;$x<$width;$x++){
for ($y=0;$y<$height;$y++){
$rgb = ImageColorAt($im, $x, $y);
$r = ($rgb >> 16) & 0xFF;
$g = ($rgb >> 8) & 0xFF;
$b = $rgb & 0xFF;

$r2=$r;
$g2=$g;
$b2=$b;

$r=$r2*$ar[0]+$g2*$ar[1]+$b2*$ar[2];
$g=$r2*$ar[2]+$g2*$ar[0]+$b2*$ar[1];
$b=$r2*$ar[1]+$g2*$ar[2]+$b2*$ar[0];

$color = imagecolorallocate($im2, $r, $g, $b);

imagesetpixel($im2,$x,$y,$color);
}
}
$final = $im2;
if($size == "Mini") {
$final = imagecreate(15,10);
imagecopyresampled($final,$im2,0,0,0,0,15,10,30,20);
}
imagejpeg($final,NULL,100);
?>

When run on a windows Windows server it outputs different colours compared to when installed on an Ubuntu server, both have the GD2 library and php5. Any ideas as to why this would happen?

 

Ok, here is some more information, I originally found the script here:

http://www.phpbuilder.com/snippet/download.php?type=snippet&id=1732

It seems the problem is during the bitshift operations, removing them both produces the same image on both servers.

 

Maybe the gd2 lib is progrmmed a little diff in ubuntu than windows cause they have diff program langs