php imagick convert PNG to jpg

Question

$image = "[...]"; //binary string containing PNG image
$file = fopen('image.tmp', 'wb');
fputs($file, $image);
fclose($file);
$image = new Imagick('PNG:image.tmp');
$image->thumbnailImage($width, $height);
$image->setImageFormat('jpg');
$image->setCompressionQuality(97);
$image->writeImage('image.jpg');

The above doesn't work and gives me a black image for this image. When doing instead

[...]
$image->setImageFormat('png');
$image->setCompressionQuality(97);
$image->writeImage('image.png');

all is fine again. I think it has to do something with transparent background, which isn't available in JPG format. Can anyone help to solve this (imagick isn't documented very well, so I don't know how to help myself).

Answer 1

Found a solution:

$white=new Imagick();
$white->newImage($width, $height, "white");
$white->compositeimage($image, Imagick::COMPOSITE_OVER, 0, 0);
$white->setImageFormat('jpg');
$white->writeImage('image.jpg');