Description:
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I want to draw many squares on a PNG image, wich have changing color.

after some correct drawed there is only one color used.

Reproduce code:
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<?php 

$bigsize 	= 500;

$colours	= array( 'FF0000' , '00FF00' );

header("Content-type: image/png");
$im 	= imagecreate( $bigsize * 2 , 50 );

for( $j = 0; $j < $bigsize ; $j++ ) {

	$k	= $j % count( $colours );
	$r	= hexdec( substr( $colours[ $k ] , 0 , 2 ) );
	$g	= hexdec( substr( $colours[ $k ] , 2 , 2 ) );
	$b	= hexdec( substr( $colours[ $k ] , 4 , 2 ) );
	$fill	= imagecolorallocate($im, $r , $g , $b );
	imagefilledrectangle( $im , 2*$j , 0 , 2*$j+1 , 50 , $fill );

}

imagepng($im);
imagedestroy($im);

?>

Expected result:
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a bar with red and green vertical stripes wich have changing colors.

Actual result:
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the right half of the bar is green.