Color-keying image not working in PHP

Question

The input image has a magenta background.

The outputted image always has a black background.

Both attempts give same results. I want it to be alpha background, not black background.

Attempt #1:

imagesavealpha($image, true);
imagealphablending($image, false);
$trans = imagecolorallocate($image, 255, 0, 255);
imagecolortransparent($image, $trans);
imagepng($image, $label . '.png');

Attempt #2:

imagesavealpha($image, true);
imagealphablending($image, false);
$trans = imagecolorallocatealpha($image, 0, 0, 0, 0);
for ($x = 0; $x < imagesx($image); ++$x) {
    for ($y = 0; $y < imagesy($image); ++$y) {
        $rgb = imagecolorat($image, $x, $y);
        $r = ($rgb >> 16) & 0xFF;
        $g = ($rgb >> 8) & 0xFF;
        $b = $rgb & 0xFF;
        if ($r == 255 && $g == 0 && $b == 255) {
            imagesetpixel($image, $x, $y, $trans);
        }
    }
}
imagepng($image, $label . '.png');

Answer 1

Seems imagecolorallocatealpha's alpha parameter is non-standard. Uses 0 to 127, where 127 is fully-transparent.

Answer 2

Your attepnt #1 looks good to me, just don't touch the alphablending.

To avoid matters with transparency, I'm used to set it just after creating my image :

$image = imagecreatetruecolor($width, $height);
$trans = imagecolorallocate($image, 0xFF, 0x0, 0xFF);
imagefill($image, 0, 0, $trans);
imagecolortransparent($image, $trans);

Here, I'm sure my whole image background is transparent and I just need to put my stuffs in it.