<?php

function sendmail_utf8($to, $subject, $msg) {
    function bw($text) {
        return '=?UTF-8?B?'.base64_encode($text).'?=';
    };
    function bo($text) {
        return base64_encode($text);
    };

    $sender_name = basename(__FILE__);

    $sender_name_en = bw($sender_name);
    $subject_en = bw($subject);
    $msg_en = bo(str_replace(PHP_EOL, "\r\n", $msg));
    return mail($to, $subject_en, $msg_en, array(
        'From' => $sender_name_en.'<'.$to.'>',
        'X-Mailer' => 'sendmail_utf8_on_php',
        'MIME-Version' => '1.0',
        'Content-Transfer-Encoding' => 'base64',
        'Content-Type' => 'text/plain; charset=UTF-8'
    ));
};

sendmail_utf8(
    'mailaddress@domainname',
    'テストメールfutaba_searchwatch',
    '送信テスト'
);