Background transperancy in imagerotate() - php

Question

Since last 2 days, I was trying to add transperancy to the background after rotating an image using imagerotate() PHP-GD function.
But, to my great disappointment, it's not working at all.
It's just giving out a black background behind it.
Here's my code -
$patchImageS = 'image.png'; // the image to be patched over the final bg
$patchImage = imagecreatefrompng($patchImageS); // resource of image to be patched
$patchImage = imagerotate($patchImage, 23, 0, 0);
imagepng($patchImage,'tt.png');
I tried to change the parameters being passed in function to
imagerotate($patchImage, 23, 5, 0);
imagerotate($patchImage, 23, 0, 5);
Any help would be highly appreciated.

Answer 1

After a number of 99% finished answers, here's the solution I've found:
// Create, or create from image, a PNG canvas
$png = imagecreatetruecolor($width, $height);
// Preserve transparency
imagesavealpha($png , true);
$pngTransparency = imagecolorallocatealpha($png , 0, 0, 0, 127);
imagefill($png , 0, 0, $pngTransparency);
// Rotate the canvas including the required transparent "color"
$png = imagerotate($png, $rotationAmount, $pngTransparency);
// Set your appropriate header
header('Content-Type: image/png');
// Render canvas to the browser
imagepng($png);
// Clean up
imagedestroy($png);
The key here is to include your imagecolorallocatealpha() in your imagerotate() call...

Answer 2

look for imagesavealpha() in the php-documentation - i think this is what you are looking for.
EDIT: here's an example:
$png = imagecreatefrompng('./alphachannel_example.png');
// Do required operations
$png = imagerotate($png, 23, 0, 0);
// Turn off alpha blending and set alpha flag
imagealphablending($png, false);
imagesavealpha($png, true);
// Output image to browser
header('Content-Type: image/png');
imagepng($png);
imagedestroy($png);

Answer 3

For anyone having problems with imagecopyresampled or imagerotate with black bars on background, I have found a code example here:
https://qna.habr.com/q/646622#answer_1417035
// get image sizes (X,Y)
$wx = imagesx($imageW);
$wy = imagesy($imageW);
// create a new image from the sizes on transparent canvas
$new = imagecreatetruecolor($wx, $wy);
$transparent = imagecolorallocatealpha($new, 0, 0, 0, 127);
$rotate = imagerotate($imageW, 280, $transparent);
imagealphablending($rotate, true);
imagesavealpha($rotate, true);
// get the newest image X and Y
$ix = imagesx($rotate);
$iy = imagesy($rotate);
//copy the image to the canvas
imagecopyresampled($destImg, $rotate, 940, 2050, 0, 0, $ix, $iy, $ix, $iy);