合并 png 图像并调整其大小

我需要合并两个.png图像并调整输出大小，我有以下代码：

public function image($bg = null, $img = null)
{
        define("WIDTH", 1200);
        define("HEIGHT", 1200);
        $dest_image = imagecreatetruecolor(WIDTH, HEIGHT);
        imagesavealpha($dest_image, true);
        $trans_background = imagecolorallocatealpha($dest_image, 0, 0, 0, 127);
        imagefill($dest_image, 0, 0, $trans_background);
        $a = imagecreatefrompng('./test/'.$bg);
        $b = imagecreatefrompng('./test/'.$img);
        imagecopy($dest_image, $a, 0, 0, 0, 0, WIDTH, HEIGHT);
        imagecopy($dest_image, $b, 0, 0, 0, 0, WIDTH, HEIGHT);
        header('Content-Type: image/png');
        imagepng($dest_image);
        imagedestroy($a);
        imagedestroy($b);
        imagedestroy($dest_image);
}

代码返回原始大小的PNG图像，如何使此函数返回其他大小的图像，例如（200 * 200px）