If your server doesn't have mb_send_mail() enabled but you want to use non-ascii (multi-byte) chars in an email's subject or name headers, you can use something like the following:

$charset = "iso-2202-jp"; // japanese
$to = encode("japanese name 01", $charset) . " <[email protected]>";
$from = encode("japanese name 02", $charset) . " <[email protected]>";
$subject = encode("japanese text", $charset);
$message = "does not need to be encoded";
mail($to, $subject, $message, $from);

function encode($in_str, $charset) {
    $out_str = $in_str;
    if ($out_str && $charset) {

        // define start delimimter, end delimiter and spacer
        $end = "?=";
        $start = "=?" . $charset . "?B?";
        $spacer = $end . "\r\n " . $start;

        // determine length of encoded text within chunks
        // and ensure length is even
        $length = 75 - strlen($start) - strlen($end);
        $length = floor($length/2) * 2;

        // encode the string and split it into chunks 
        // with spacers after each chunk
        $out_str = base64_encode($out_str);
        $out_str = chunk_split($out_str, $length, $spacer);

        // remove trailing spacer and 
        // add start and end delimiters
        $spacer = preg_quote($spacer);
        $out_str = preg_replace("/" . $spacer . "$/", "", $out_str);
        $out_str = $start . $out_str . $end;
    }
    return $out_str;
}
// for details on Message Header Extensions 
// for Non-ASCII Text see ...
// http://www.faqs.org/rfcs/rfc2047.html

# self-fix...
I posted "encode_mimeheader" workaround on the day before. But I found that the code depends on platforms. :(

In some platforms, hedders(after the header splited into two or more lines) will appear in the body content.
The cause is that 'there are some platforms where the translation from \n to \r\n is apparently done for you'.
# See the post by "leon at ietsmet dot nl" about mail function (23-Apr-2003).

Now, all you have to do is just changing the glue string from "\r\n " to "\n ".

function encode_mimeheader($str, $indent = 0, $encoding = 'utf-8', $mail_encoding = 'iso-2022-jp')
{
         ..... lefted out (all same).....
  $str = join("\r\n ", $lines); // RFC 2047,822 say newline must be ^\r\n, not only\n
  return $str;
}

should be below in some platform...

function encode_mimeheader($str, $indent = 0, $encoding = 'utf-8', $mail_encoding = 'iso-2022-jp')
{
         ..... lefted out (all same).....
  $str = join("\n ", $lines);
  // Though RFC 2047,822 say newline must be \r\n, not only \n,
  // in my platform, auto replacement will be done (accepts \n and \n\r\n -> \n\n)...
  return $str;
}

# notice: In both code, 1 space after newline is essential (gule str is *not* only "\r\n" nor "\n").
# This space is ignored only between encoded-words.

    c.f. about efficiency

Thinking about efficiency, my encode_mimeheader may have to determine where to split string into lines without mb_encode_mimeheader in the determining loop. But that loop will be repeated only one or a few times and this function  will be usually used against not so long string. So, this loss is not critical in most case.
If you use only one encoding, you can easily check ascii or multi-byte char one by one accoding to the definition of the encoding and determining splitting point (this is far faster), but when you have to handle all encoding that is not so easy. At least, I don't want to touch. :p
To accept *any* encoding (encoding name will be passed as argument), you can use this code. This is because I post non-efficient code like this.
# Though I'm not sure whether the code actually work well with all encoding...

I have to use Japanese hankaku (single byte kana in (S)JIS) in mail subject and message,
so I tried the way that NO3L in Japan said, but I found there's a big problem.
In that way, mb_encode_mimeheader drop escape sequence from the head of encoded string from second line.
if you use long letter in subject, and if it includes multi byte string, It must crashed.
Therefore, if you want to use Japanese hankaku in mail subject, you might try like this.
Encoded subject letters in one line is not just 76 letters, so it is not based on RFC, but it work.

$intSubjectLength  = mb_strlen($strSubject);
$intSeparateLength = 10;
for ($i=0; $i<ceil($intSubjectLength / $intSeparateLength); $i++) {
    $arrSeparatedSubject[$i] = mb_substr($strSubject, $intIndex, $intSeparateLength);
    $arrSeparatedSubject[$i] = mb_encode_mimeheader(mb_convert_encoding($arrSeparatedSubject[$i], "JIS", "EUC-JP"));
    $intIndex = $intIndex + $intSeparateLength;
}
$strSubject = join("\n ", $arrSeparatedSubject);

Make sure that if you're using a form to type in the mail, that your form page has the right encoding, like if I want to send out a japanese email, by filling out a form, the form page needs this in the header:
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=SHIFT-JIS">

I was trying to send japanese email and had a hell of a time getting it to work. I finally stumbled across mb_send_mail() and it did everything I wanted *except* that it only send plain text emails ... sending HTML contents was impossible.

I finally whipped this up. I'm posting in case someone finds it useful.

function send_japanese_mail($to, $subject, $body, $from, $from_email, $is_html_content=false) {

   $headers  = "MIME-Version: 1.0\\n" ;
   $headers .= "From: $from <$from_email>\\n";
   $headers .= "Reply-To: $from <$from_email>\\n";
 
  /* If the body  is HTML or plain text set the Content-Type header accordingly */

   if ($is_html_content) {
     $headers .= "Content-Type: text/html;charset=ISO-2022-JP\\n";
     /* turn all line breaks into BR tags */
     $body = nl2br($body);
   }
     else {
     $headers .= "Content-Type: text/plain;charset=ISO-2022-JP\\n";
   }

  /* need to convert body to same encoding as stated in Content-Type header above */

  $body = mb_convert_encoding($body, "ISO-2022-JP","AUTO");

  /* set any sendmail params, optional ... */
  $sendmail_params  = "-f$from_email";

  /*
   The subject is actually a "header" and can/will get mangled
   if it contains non-ASCII characters. So we need to convert
   the subject to something containing only ASCII characters.

   First we convert the subject to the same encoding as the
   body, then we use mb_encode_mimeheader() to make the subject
   line all ASCII characters.
  */

   mb_language("ja");
   $subject = mb_convert_encoding($subject, "ISO-2022-JP","AUTO");
   $subject = mb_encode_mimeheader($subject);

   mail($to, $subject, $body, $headers, $sendmail_params);
}

Tested with PHP 4.2.2 on Linux: Please note that if you're using Unicode (mb_language("uni")) and you attempt to send mail with mb_send_mail(), you will need to base64_encode() the message body - mb_send_mail() doesn't do that for you. It does, however, issue the correct message headers, so you don't need to worry about that.

Also note that neither mb_language() nor mb_send_mail() is able to convert your message to UTF-8. *You* need to provide the UTF-8-encoded (and base64-encoded) message, and then mb_send_mail() will issue the correct message headers.

Here's an example of sending an UTF-8-encoded message with mb_send_mail():

mb_language("uni");
$body = chunk_split(base64_encode("International characters"));
mb_send_mail("[email protected]", "Subject", $body);

If the receiving mail client supports UTF-8 properly, you will be able to send messages that contain a mix of all kinds of characters (e.g., you could send Thai, Chinese, and Danish characters in the same message). Not all mail clients support UTF-8, though. At the time of writing, some of the more popular Windows email clients - Eudora and Pegasus Mail - don't. There are a number of email clients with working support for UTF-8. These include Outlook Express, KMail, Mozilla, Netscape 6/7, Sylpheed, Evolution and others.

If you have problem using mb_send_mail to send utf-8 mails, you have to know the following things (I've spent hours on the net before finding the correct php code) :

- You have to use the mb_encode_mimeheader function on the sender name and on the recipient name (on names, not on emails !).
- Subject and message are automatically encoded.
- Add - at least - the following header :

$headers = "Mime-Version: 1.0\n";
$headers .= "Content-Type: text/plain;charset=UTF-8\n";
$headers .= "From: $sender";

- Last but not least, beware of the internal encoding, which is needed by mb_encode_mimeheader function in order to encode properly. I had to set the internal encoding to UTF-8 in order to make it work properly :

mb_internal_encoding("UTF-8");

While using the right tags mentioned in other comments (mb_internal_encoding, Content-Type, etc) ...

mb_send_mail would properly encode special characters in plain text emails, for both subject and message

mb_send_mail would not properly encode special characters in subject for multi-part messages (text+html)

Easy and simple....
Send an email with Japanese contain.

<?php

/**
     * @name                    : sendMail
     * @author Taslim Mazumder Sohel
     * @[email protected]
     * Function for sending email 
     *with Japanese Email Body, Subject, Sender Name.
     *
     * @param String $to               : Receiver mail address.
     * @param String $subject          : email subject.
     * @param int    $body            : mail body text.
     * @param array  $from_email    : Sender mail address.
     * @param array  $from_name      : Sender Name.
     *
     */
function sendMail($to, $subject, $body, $from_email,$from_name)
 {
$headers  = "MIME-Version: 1.0 \n" ;
$headers .= "From: " .
       "".mb_encode_mimeheader (mb_convert_encoding($from_name,"ISO-2022-JP","AUTO")) ."" .
       "<".$from_email."> \n";
$headers .= "Reply-To: " .
       "".mb_encode_mimeheader (mb_convert_encoding($from_name,"ISO-2022-JP","AUTO")) ."" .
       "<".$from_email."> \n";
 
    
$headers .= "Content-Type: text/plain;charset=ISO-2022-JP \n";

    
/* Convert body to same encoding as stated 
in Content-Type header above */
    
$body = mb_convert_encoding($body, "ISO-2022-JP","AUTO");
    
/* Mail, optional paramiters. */
$sendmail_params  = "-f$from_email";
    
mb_language("ja");
$subject = mb_convert_encoding($subject, "ISO-2022-JP","AUTO");
$subject = mb_encode_mimeheader($subject);

$result = mail($to, $subject, $body, $headers, $sendmail_params);
       
return $result;
}
        
?>

There is another way of encoding Japanese language to send email without worrying about mb_functions:

Download PHP Jcode from http://www.spencernetwork.org/
and use JcodeConvert function with base64_encode function for the e-mail header and the subject.

$email_to = "[email protected]";

$_POST["name"] = strip_tags(trim($_POST["name"]));
$name = "=?ISO-2022-JP?B?". base64_encode(JcodeConvert($_POST["name"], 2, 3)). "?=";
$_POST["email"] = strip_tags(trim($_POST["email"]));
$_POST["subject"] = strip_tags(trim($_POST["subject"]));
$email_subject = "=?ISO-2022-JP?B?". base64_encode(JcodeConvert($_POST["subject"], 2, 3)). "?=";
$_POST["body"] = strip_tags(trim($_POST["body"]));

$email_header = "From: $name <". $_POST["email"] .">\n";
$email_header .= "Reply-To: ". $_POST["email"] ."\n";
$email_header .= "Content-transfer-encoding: 7bit\n";
$email_header .= "Content-type: text/plain; charset=\"iso-2022-jp\"\n\n";

$email_body = "blah blah blah.\n"

mail($email_to, $email_subject, $email_body, $email_header);

Be careful, mb_send_mail chokes if you try and send a multipart e-mail.

You may not use mb_encode_mimeheader() with mb_convert_encoding() to make subject as follows. It causes mojibake in several strings.
mb_encode_mimeheader( mb_convert_encoding($strMailSubj, "JIS", "EUC-JP") )

Set mb_internal_encoding() to *subject's* encoding and call mb_encode_mimeheader.

Example)

    $__lang = mb_language();
    $__enc = mb_internal_encoding();
    mb_language("Japanese");
    mb_internal_encoding( mb_detect_encoding($subject) );
    #mb_internal_encoding( "EUC-JP" ); #just do when you know encoding of $subject
    mail($to,
        mb_encode_mimeheader($subject),
        mb_convert_encoding($msg,"JIS","AUTO"),$header);
    mb_internal_encoding( $__enc );
    mb_language($__lang);

As others say, mb_encode_mimeheader() seems to have a bug with JIS(ISO-2022-JP) encording. Token indicating start and end of multibyte char is inserted only before start of encoding and after the all.
We need the tokens at start and end of *all* encoded-text.
# PHP 4.3.2

So, we needs some workaround.

The post by "gordon at kanazawa-gu dot ac dot jp" seems to work well, but it doesn't. JIS nees some special tokens and base64_encode does'nt output them, so the code cannot used.

The post "RE: N03L in Japan", which simply splitting words in eash 10 chars is good enough in most case, there are some problem left. When there are some splited parts starting with ascii chars, 1 additional space will be inserted.
# RFC2047 only say within 76 words, shorter is not bad.

Additionally, thought it is really rare case, when we use "=?charset?" as literal string, we have to escape them.
# Some mailer can actually send this without escape and header will be broken. :(

Now, a little non-smart but maybe more accurate code is below function. I tested only with ISO-2022-JP, only in costomized phpBB2.0.5, only some cases.
As far as my test this code worked well, but I'm not sure.

# $str: source text
# $indent: ex. for "Subject: " header, give 9 and first line will be shorter than 76-1-9=66
# $encoding: source text encoding
# $mail_encoding: $str will be converted into this before base64 encode

function encode_mimeheader($str, $indent = 0, $encoding = 'utf-8', $mail_encoding = 'iso-2022-jp')
{
  $start_delimiter = strtoupper("=?$mail_encoding?B?");
  $start_pattern   = strtoupper("=\\?$mail_encoding\\?B\\?");
  $end_delimiter   = '?=';

  $str = mb_convert_encoding($str, $mail_encoding, $encoding);
  $length  = mb_strlen($str, $mail_encoding);
  $max_part_length = 20; // enough short in most case (you can change this default value)
    
  for ($i=0, $index=0; $index<$length; $i++) {
    $part_length = $max_part_length;
    $s = mb_substr($str, $index, $part_length, $mail_encoding);
    // workaround for literally used start delimiter (without below, subject may break)
    // note: mb_encode_mimeheader() don't encode ascii including start delimiter
    if (preg_match('/^' . $start_pattern . '/i', $s))
    {
      $lines[$i] = $start_delimiter . base64_encode($start_delimiter) . "?=";
      $index += strlen($start_delimiter);
      continue;
    }

    $lines[$i] = mb_encode_mimeheader($s, $mail_encoding);
    while (strlen($lines[$i]) > 76-1 - ($i?0:$indent)) { // max par line - first space - indent (first line only)
      $part_length = floor($part_length * (76-1 - ($i?0:$indent)) / strlen($lines[$i])); // at least 1 decrement
      $s = mb_substr($str, $index, $part_length, $mail_encoding);
      $lines[$i] = mb_encode_mimeheader($s, $mail_encoding);
    }
    
    // workaround for starting new line with ascii (without below, 1 space may cut in)
    // note: mb_encode_mimeheader() starts encode after encounterring multibyte char
    if ($i > 0 && !preg_match('/^' . $start_pattern . '/i', $lines[$i]))
    {
      $p = strpos($lines[$i], $start_delimiter); //never 0 (false when not found)
      $p = $p ? $p : strlen($lines[$i]);
      $lines[$i] = $start_delimiter . base64_encode(substr($lines[$i], 0, $p)) . "?=";
      $part_length = $p;
    }
    $index += $part_length;
  }
  $str = join("\r\n ", $lines); // RFC 2047,822 say newline must be ^\r\n, not only\n
  return $str;
}

First of all, don't use nl2br in newer php versions unless you WANT to write xhtml...you'll get <br /> tags.

Secondly, you should use JIS, not ISO-2022-JP as the convert TO encoding.  If you use ISO... mb_encode_mimeheader seems to fail. They're the same charset anyway.

Now here's why I am really writing.  I just discovered that mb_send_mail and mb_encode_mimeheader cannot support hankaku (single byte kana in (S)JIS) at all.  If you are making apps for the keitai (mobile) market, this won't do.  Therefore, undo your overloading on the mail function and use regular mail and convert everything yourself like this.  This is what we did.  It seems to work for us.  Your mileage may vary.

$strMBS = mb_convert_encoding($strMailBodySend, "JIS", "EUC-JP");

$strMS  = mb_encode_mimeheader( mb_convert_encoding($strMailSubj, "JIS", "EUC-JP") );

mail($strToMail, $strMS, $strMBS, $strFrom);