If you need lower-case header-names (as I read the documentation case is not guranteed) try something like
<?php
$headers_string = <<<EOF
Subject: =?UTF-8?B?UHLDvGZ1bmcgUHLDvGZ1bmc=?=
To: [email protected]
Date: Thu, 1 Jan 1970 00:00:00 +0000
Message-Id: <[email protected]>
Received: from localhost (localhost [127.0.0.1]) by localhost
with SMTP id example for <[email protected]>;
Thu, 1 Jan 1970 00:00:00 +0000 (UTC)
(envelope-from [email protected])
Received: (qmail 0 invoked by uid 65534); 1 Thu 2003 00:00:00 +0000
EOF;
$headers = iconv_mime_decode_headers($headers_string, 0, "ISO-8859-1");
$headers = array_combine(array_map("strtolower", array_keys($headers)), array_values($headers));
print_r($headers);
?>