[Mail] 请教邮件utf8编码的问题

[weixin_38058939]

??收到一封邮件头如下：
From: =?utf-8?B?5byg5qyi?= <[e-mail]>;
Subject: Fw: INTELLIGENT WAVE
Message-id: <002801c33fa5$a8fa5e40$[e-mail]>;
MIME-version: 1.0
X-MIMEOLE: Produced By Microsoft MimeOLE V6.00.2600.0000
X-Mailer: Microsoft Outlook Express 6.00.2600.0000
Content-type: text/plain; charset=utf-8
Content-transfer-encoding: base64
X-Priority: 3
X-MSMail-priority: Normal

??不知道怎么用PHP解码，用base64_decode()不行，utf8_decode()也不行，结合起来utf8_decode(base64_decode())也不行，请教各位长老！