Why does saving the alpha channel in GD not work?

Question

PHP

It is necessary to save the alpha channel of the image (.png).
Tried this code:

$up = ImageCreateFrompng('up.png');
imagealphablending($up, false);
imagesavealpha($up, true);
imagecopy($pic, $up, 200, 200, 0, 0, 145, 343);

The image is still superimposed with a white background, what could be the problem?

Answer 1

I solved the problem by the fact that it was necessary to save both images in PNG-24.

Answer 2

Probably need to include alpha in $pic before this

$up = ImageCreateFrompng('up.png');
$pic = imagecreatetruecolor(400, 400);
imagealphablending($pic, false);
// imagesavealpha($pic, true); // true alpha channel
imagecopy($pic, $up, 200, 200, 0, 0, 145, 343);//, 145, 343
imagesavealpha($pic, true); // PNG-24
imagepng($pic, 'pic.png');