Decode base64 string - php

Question

Is there any way to decode this string?

Actual string: 其他語言測試 - testing.

base64 encode while sending on mail as subject as:

"=?iso-2022-jp?B?GyRCQjZCPjhsOEBCLDtuGyhCIC0gdGVzdGluZw==?="

<?php
echo base64_decode('GyRCQjZCPjhsOEBCLDtuGyhCIC0gdGVzdGluZw==');
?>

This is a base 64 encode, I couldn't decode it to an actual Chinese string. Since it has been encoded using "iso-2022-jp", I have also tried the online base64decode.org site to decode this string, but I couldn't find the original string, how can I do that?