Scaling image is losing background transparency


I'm using PHP 5 and trying to scale png images programatically. Here
is what I'm doing ...

$src_img = imagecreatefrompng($imagefile);
$hw = getimagesize($imagefile);
$new_w = $w;
$new_h = $hw["1"]/($hw["0"]/$w);
$dst_img = @imagecreatetruecolor($new_w, $new_h);
if(!$dst_img) {
$dst_img = imageCreate($new_w, $new_h);

The problem is my scaled png image loses its background transparency.
The short of this is, does anyone know how to scale PNG images
effectively and what other information can I provide to help
troubleshoot this further?

Thanks, - Dave