[PHP] mb_decode_mimeheader() は Quoted-printable なヘッダのデコードに失敗する ref: http://qiita.com/ryo511/items/82022f836f0fe033020b

file0.txt

=?utf-8?Q?=E3=83=86=E3=82=B9=E3=83=88This_is_valid_subject_of_MIM?=
 =?utf-8?Q?E_header?=

file1.php

<?php

$subject = <<<TXT
=?utf-8?Q?=E3=83=86=E3=82=B9=E3=83=88This_is_valid_subject_of_MIM?=
 =?utf-8?Q?E_header?=
TXT;

var_dump(mb_decode_mimeheader($subject)); // string(45) "テストThis_is_valid_subject_of_MIME_header"
var_dump(imap_utf8($subject)); // string(45) "テストThis is valid subject of MIME header"
var_dump(decode_mime_header($subject)); // string(45) "テストThis is valid subject of MIME header"

function decode_mime_header($str) {
    $decoded = '';
    $parts = imap_mime_header_decode($str);
    foreach ($parts as $part) {
        $decoded .= $part->text;
    }
    return $decoded;
}

file2.php

function decode_mimeheader($str) {
    return mb_decode_mimeheader(str_replace('_', '=20', $str));
}

file2.txt

function decode_mimeheader($str) {
    return mb_decode_mimeheader(str_replace('_', '=20', $str));
}