barts code below does not work at least with gd 2
Only returns a blank image with alpha not the source resized
$im = ImageCreateFromPNG($sourcefile);
$im_dest = imagecreatetruecolor ($forcedwidth, $forcedheight);
imagealphablending($im_dest, false);
imagecopyresampled($im_dest, $im, 0, 0, 0, 0, $wm_width, $wm_height, $forcedwidth, $forcedheight);
imagesavealpha($im_dest, true);
imagepng($im_dest, $destfile);
imagedestroy($img_dest);
ps you also forgot image destroy and you had a random var in imagepng undefined in your post
