Want to save png to file

Hi! Creating a transparent png using GD and PHP is (almost) no problem, but it's impossible to save… No matter what I try, the saved image wont remember that it's transparent. I suspect the reason it works "on the fly" is because I manually set the header to image/png, but if I try so save the stream to a file, the transparency disappears… Please please please somebody help! Sincerely Victor