为什么urlencode数据要通过curl发布，为什么不对表单做同样的事情呢？

As stated on php manual urlencode is for encoding query part of url, so why should urlencode be used to encoded data before sending via curl too as these are $_POST values and not query part?

    foreach ($data as $key => $value) {
             $value = urlencode($value);
             $req .= "&$key=$value";
    }
    //
    curl_setopt($ch, CURLOPT_POSTFIELDS, $req);

If this is recommended to do, so why this is not necessary to encode posted data via form before processing a submitted form (not for curl, but just processing form I mean.) ?

Does this mean curl is sending data differently than a submitted form does?

You can do:

curl_setopt($ch, CURLOPT_POSTFIELDS, $data);

instead of the foreach loop. When you give an array, the parameters are posted using multipart/form-data encoding.

You shouldn't call urldecode in the processing form, because PHP automatically decodes the parameters before putting them into $_GET or $_POST.