如何使用PHP将文件.png转换为.CUBE

我找不到解决这个问题的代码，也许有人可以帮我，我想让file.png上传，然后我按生成按钮，它将是.CUBE并自动下载它

请检查下面的代码，它对我有效

$fileupload = $_FILES['fileupload']['name'];
$fileName = pathinfo($fileupload)['filename'];
$fileExtension=pathinfo($fileupload)['extension'];
if($fileExtension=="png")
{
    $fileupload =  $fileName.".CUBE";
}
$filePathURL=$fileupload;
move_uploaded_file($_FILES['fileupload']['tmp_name'], "$filePathURL");
header("Cache-Control: private");
header("Content-Type: application/stream");
header("Content-Disposition: attachment; filename=".$fileupload);