Description:
------------
A mail message fetched from gmail. The original message encoded in 
big5, but the result come from gmail become GB2312 in attachments' 
name. And when converting it into utf8 with mb_convert_encoding(), the 
result become unreadable. But when change GB2312 to GBK, the result 
becomes better. Although I'm not sure if it still correct in 
characters' code, but I could read it.

Hope you won't mind that I only tested it in PHP 5.2.6.

Reproduce code:
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<?php
$encodedString = '=?GB2312?B?MjAwOTExMDZfQ1lDVU1JU0xhYl+5UNObX5pXwM+On9Ob5JsuZG9j?=';
$imapDecode = imap_mime_header_decode($string);
// Returning unreadable result
mb_convert_encoding($imapDecode[0]->text, 'UTF-8', 'GB2312');
// Returning correct result
mb_convert_encoding($imapDecode[0]->text, 'UTF-8', 'GBK');