我使用此代码在图像上添加水印。问题是，当我更改$sx，$sy时，它会剪切水印，而不会更改大小。
我该怎么办？
守则：

$sx = imagesx($stamp);
    $sy = imagesy($stamp);

    $des_width = imagesx($im);
    $des_height = imagesy($im);

    $xpos = $des_width - $sx - $marge_right;
    $ypos = $des_height - $sy - $marge_bottom;

    $trans_background = imagecolorallocatealpha($im, 0, 0, 0, 127);

    $dest_image = imagecreatetruecolor($des_width, $des_height);
    imagesavealpha($dest_image, true);

    $trans_background = imagecolorallocatealpha($dest_image, 0, 0, 0, 127);

    imagefill($dest_image, 0, 0, $trans_background);

    imagecopy($dest_image, $im, 0, 0, 0, 0, $des_width, $des_height);
    imagecopy($dest_image, $stamp, $xpos, $ypos, 0, 0, $sx, $sy);

    if(pathinfo($file, PATHINFO_EXTENSION) == 'png'){
        imagepng($dest_image, $file);
    } else {
        imagejpeg($dest_image, $file, 99);
    }

    imagedestroy($stamp);   
    imagedestroy($im);
    imagedestroy($dest_image);