function encodeMB($str, $charset, $encoding, $prefix_len=0, $eol="\r\n")
    {
        if (!function_exists('mb_substr') || !function_exists('mb_strlen')) {
            return;
        }

        $encoding = $encoding == 'base64' ? 'B' : 'Q';
        // 75 is the value specified in the RFC
        $prefix = '=?' . $charset . '?'.$encoding.'?';
        $suffix = '?=';
        $maxLength = 75 - strlen($prefix . $suffix);

        // A multi-octet character may not be split across adjacent encoded-words
        // So, we'll loop over each character
        // mb_stlen() with wrong charset will generate a warning here and return null
        $length      = mb_strlen($str, $charset);
        $result      = '';
        $line_length = $prefix_len;

        if ($encoding == 'B') {
            // base64
            $start = 0;
            $prev  = '';

            for ($i=1; $i<=$length; $i++) {
                // See #17311
                $chunk = mb_substr($str, $start, $i-$start, $charset);
                $chunk = base64_encode($chunk);
                $chunk_len = strlen($chunk);

                if ($line_length + $chunk_len == $maxLength || $i == $length) {
                    if ($result) {
                        $result .= "\n";
                    }
                    $result .= $chunk;
                    $line_length = 0;
                    $start = $i;
                } else if ($line_length + $chunk_len > $maxLength) {
                    if ($result) {
                        $result .= "\n";
                    }
                    if ($prev) {
                        $result .= $prev;
                    }
                    $line_length = 0;
                    $start = $i - 1;
                } else {
                    $prev = $chunk;
                }
            }
        } else {
            // quoted-printable
            // see encodeQP()
            $regexp = '/([\x22-\x29\x2C\x2E\x3A-\x40\x5B-\x60\x7B-\x7E\x80-\xFF])/';

            for ($i=0; $i<=$length; $i++) {
                $char = mb_substr($str, $i, 1, $charset);
                // RFC recommends underline (instead of =20) in place of the space
                // that's one of the reasons why we're not using iconv_mime_encode()
                if ($char == ' ') {
                    $char = '_';
                    $char_len = 1;
                } else {
                    $char = preg_replace_callback(
                        $regexp, array('Mail_mimePart', '_qpReplaceCallback'), $char
                    );
                    $char_len = strlen($char);
                }

                if ($line_length + $char_len > $maxLength) {
                    if ($result) {
                        $result .= "\n";
                    }
                    $line_length = 0;
                }

                $result      .= $char;
                $line_length += $char_len;
            }
        }

        if ($result) {
            $result = $prefix
                .str_replace("\n", $suffix.$eol.' '.$prefix, $result).$suffix;
        }

        return $result;
    }