Executing `createimagefrompng()` from save_post hook (or equivalent hook)

I’m having trouble executing createimagefrompng() from a hook. I’ve used the fairly straightforward PHP way to add a watermark to an image. If I hardcode the image URLs in this function and for example execute it from a page or whatever it seems to work flawlessly, but whenever I execute the exact function from a hook (to add watermarks to images just uploaded to a post), WordPress seems to ‘crash’ (as in, the loading spinner keeps spinning and nothing actually happens).

The PHP code I use for the watermark – which works perfectly when called from a template or whatever:

// Load the stamp and the photo to apply the watermark to
$stamp = imagecreatefrompng('stamp.png');
$im = imagecreatefromjpeg('photo.jpeg');

// Set the margins for the stamp and get the height/width of the stamp image
$marge_right = 10;
$marge_bottom = 10;
$sx = imagesx($stamp);
$sy = imagesy($stamp);

// Copy the stamp image onto our photo using the margin offsets and the photo 
// width to calculate positioning of the stamp. 
imagecopy($im, $stamp, imagesx($im) - $sx - $marge_right, imagesy($im) - $sy - $marge_bottom, 0, 0, imagesx($stamp), imagesy($stamp));

// Output and free memory
header('Content-type: image/png');
imagepng($im);
imagedestroy($im);

Whenever I put this in a function, and comment out code part by part it seems to already crash on the first $stamp = ... line. Can’t really figure out why this happens.

tl;dr scenario sketch
I want to add a watermark to every image uploaded in an ACF field in a specific posttype but the watermark PHP code provided in the PHP.net docs crashes on the first line of code.

Thanks guys!

The use of header() function produce output in the middle of the request and crash it.

To modify uploaded image you should use wp_handle_upload filter instead of save_post action. Also, ideally, you should use WP_Image_Editor class, which will use the best image manipulation library available in the server (GD, Imagemagick) and triggers several filters and actions that can be used by another plugins:

add_filter( 'wp_handle_upload', 'cyb_handle_upload_callback' );
function cyb_handle_upload_callback( $data ) {

    // get instance of WP_Image_Editor class
    $image = wp_get_image_editor( $data['file'] );

    if( ! is_wp_error( $image ) ) {

        // Manipulate your image here
        $stamp = imagecreatefrompng( 'path/to/stamp.png' );
        $margin_right = 10;
        $margeing_bottom = 10;
        $sx = imagesx( $stamp );
        $sy = imagesy( $stamp );

        imagecopy(
            $data['file'],
            $stamp,
            imagesx( $data['file'] ) - $sx - $margin_right,
            imagesy( $data['file'] ) - $sy - $margin_bottom,
            0,
            0,
            imagesx( $stamp ),
            imagesy( $stamp )
        );

        // Save the modified image
        $image->save();

    }

    return $data;
}

If we follow this implementation extending WP_Editor_Image class:

add_filter( 'wp_handle_upload', 'cyb_handle_upload_callback' );
function cyb_handle_upload_callback( $data ) {

    $image = wp_get_image_editor( $data['file'] );

    if ( ! is_wp_error( $image ) && is_callable( [ $image, 'stamp_watermark' ] ) ) {

        $stamp = imagecreatefrompng( '/path/to/stamp.png' );
        $success = $editor->stamp_watermark( $stamp );

        if ( ! is_wp_error( $success ) ) {
            $image->save();
        }

    }

    return $data;
}