Imagick set background colour

I've try to use imagick to convert a JPG to PNG, but I need to apply a white background so that the png doesn't remain transparent.

I can't seem to figure it out.

I've attached the code I've got so far.

$thumb_img->thumbnailImage($newthumbwidth,$newthumbheight);
		$thumb_img->setImageBackgroundColor(new ImagickPixel('white')); 
		$thumb_img->setImageColorspace(Imagick::COLORSPACE_SRGB);
		$thumb_img->setImageFormat("jpeg");		
		$thumb_img->writeImage($folder . 'thumb_' . $destination);			
		$thumb_img->destroy();

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