Use  iconv_mime_decode();

<?php
// This yields "Subject: Prüfung Prüfung"
echo iconv_mime_decode("Subject: =?UTF-8?B?UHLDvGZ1bmcgUHLDvGZ1bmc=?=",
                       0, "ISO-8859-1");
?>

http://php.net/manual/de/function.iconv-mime-decode.php