How to set background color with GD

Hi, I am creating thumbnails with PHP, the problem is when I upload a transparent image, it makes a really bad thumbnail.
I would like to set a solid color as the background, how do I do this?
My thumbnail code is:

function thumbnail($imagename) {
list($width, $height, $type) = getimagesize("photos/$imagename");
if ($type == 1) $img = imagecreatefromgif("photos/$imagename");
else if ($type == 2) $img = imagecreatefromjpeg("photos/$imagename");
else if ($type == 3) $img = imagecreatefrompng("photos/$imagename");
if ($width > $height && $width >= 150) {
$diff = $width/150;
$width = 150;
$height = $height/$diff;
}
else if ($width < $height && $height >= 150) {
$diff = $height/150;
$height = 150;
$width = $width/$diff;
}
$dstimg = imagecreate($width,$height);
imagecopyresized($dstimg,$img, 0,0,0,0,$width,$height,imagesx($img),imagesy($img));
if ($type == 1) $img = imagegif($dstimg, "thumbnails/$imagename");
else if ($type == 2) $img = imagejpeg($dstimg, "thumbnails/$imagename");
else if ($type == 3) $img = imagepng($dstimg, "thumbnails/$imagename");
}

PHP:

 

To fill area, use imagefilledrectangle($dstimg,0,0,width,height,$bgcolor);