php GD create a transparent png image

I'm trying to create a transparent png image and layer various other pngs and jpgs to create a final png with transparency. I'm having trouble creating my initial empty transparent png. It currently has a white background.

Can anyone point me in the right direction. This is my code so far...

$image = imagecreatetruecolor(485, 500);
imagealphablending($image, false);
imagesavealpha($image, true);
$col=imagecolorallocatealpha($image,255,255,255,127);
imagefill($image, 0, 0, $col);
//imagefilledrectangle($image,0,0,485, 500,$col);


/* add door glass */
$img_doorGlass = imagecreatefrompng("glass/$doorStyle/$doorGlass.png");     
imagecopyresampled($image, $img_doorGlass, 106, 15, 0, 0, 185, 450, 185, 450);              


/* add door */
$img_doorStyle = imagecreatefrompng("door/$doorStyle/$doorStyle"."_"."$doorColor.png");     
imagecopyresampled($image, $img_doorStyle, 106, 15, 0, 0, 185, 450, 185, 450);


$fn = md5(microtime()."door_builder").".png";

if(imagepng($image, "user_doors/$fn", 1)){
  echo "user_doors/$fn";
}       
imagedestroy($image);