Executing `createimagefrompng()` from save_post hook (or equivalent hook)
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 … Read more