解码mime头保留â

I am using the PHP IMAP library to parse emails and store them in a DB. I ran across some emails with encoded subject lines. I've tried various ways to decode it, but every time it retains the 'â' instead of an apostrophe. Here are the functions I've run the encoded string through:

• imap_mime_header_decode
• iconv_mime_decode
• mb_decode_mimeheader
• imap_utf8
• imap_base64

Here's a sample of the encoded string:

=?UTF-8?B?Rnc6IFN1cHBvcnQgU1VOWeKAmXMgMjAxMSBCdWRnZXQgUmVxdWVzdA==?=