Description:
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ImagePNG does not output transparent PNG images when in true-color mode, but does when in paletted mode. Before this gets thrown in the bogus pile as a browser issue - it's not - please download both these files and open them in Photoshop and see whether or not the transparency is recognized.

This issues was previously addressed (http://bugs.php.net/bug.php?id=22323) but appears to be re-broken and still not fixed in the latest version (4.3.6)

Reproduce code:
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$Image = ImageCreateTrueColor(80,40);

$back  = ImageColorAllocate($Image,255,255,  0);
$red   = ImageColorAllocate($Image,255,  0,  0);
$green = ImageColorAllocate($Image,  0,255,  0);
$black = ImageColorAllocate($Image,  0,  0,  0);

ImageColorTransparent($Image, $back);
ImageFilledRectangle($Image,0,0,80,40,$back);
ImageArc($Image,20,20,32,32,120,0, $red);
ImageString($Image,5,20,20, 'hello', $black);

//ImageTrueColorToPalette($Image, true, 256);
ImagePNG($Image, 'c:/bad.png');


Expected result:
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I expect this:
http://www.silisoftware.com/temp/imagepng-transparency/good.png

A red arc, black text on a transparent background. And I do get this if I use the commented-out line above to convert the true color image to a paletted image.

Actual result:
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I get this:
http://www.silisoftware.com/temp/imagepng-transparency/good.png

A red arc, black text on a yellow (non-transparent) background.