php imagick convert PNG to jpg

$image = "[...]"; //binary string containing PNG image
$file = fopen('image.tmp', 'wb');
fputs($file, $image);
fclose($file);
$image = new Imagick('PNG:image开发者_开发问答.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).

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');

Another way to convert transparent png to jpg, as mentioned in Imagick::flattenImages:

$im = new Imagick('image.png');
$im->setImageBackgroundColor('white');

$im->flattenImages(); // This does not do anything.
$im = $im->flattenImages(); // Use this instead.

$im->setImageFormat('jpg');
$im->writeImage('image.jpg');

As time moves on, flattenImages() has been deprecated.
Instead of the line above use:

$im->mergeImageLayers(Imagick::LAYERMETHOD_FLATTEN);

You can use setBackgroundColor to set the default background color to something else than black. The PNG transparency will be replaced by the background color when saving to JPG.

Edit: Use it like so:

$img->setBackgroundColor(new ImagickPixel('#FFFFFF'));

Try adding $image->setBackgroundColor(0xFFFFFF); after $image = new Imagick('PNG:image.tmp');