src/Mail/mimePart.php
function encodeHeader($name, $value, $charset='ISO-8859-1',
        $encoding='quoted-printable', $eol="\r\n"
    ) {
        // Structured headers
        $comma_headers = array(
            'from', 'to', 'cc', 'bcc', 'sender', 'reply-to',
            'resent-from', 'resent-to', 'resent-cc', 'resent-bcc',
            'resent-sender', 'resent-reply-to',
            'mail-reply-to', 'mail-followup-to',
            'return-receipt-to', 'disposition-notification-to',
        );
        $other_headers = array(
            'references', 'in-reply-to', 'message-id', 'resent-message-id',
        );

        $name = strtolower($name);

        if (in_array($name, $comma_headers)) {
            $separator = ',';
        } else if (in_array($name, $other_headers)) {
            $separator = ' ';
        }

        if (!$charset) {
            $charset = 'ISO-8859-1';
        }

        // Structured header (make sure addr-spec inside is not encoded)
        if (!empty($separator)) {
            // Simple e-mail address regexp
            $email_regexp = '([^\s<]+|("[^\r\n"]+"))@\S+';

            $parts = Mail_mimePart::_explodeQuotedString($separator, $value);
            $value = '';

            foreach ($parts as $part) {
                $part = preg_replace('/\r?\n[\s\t]*/', $eol . ' ', $part);
                $part = trim($part);

                if (!$part) {
                    continue;
                }
                if ($value) {
                    $value .= $separator==',' ? $separator.' ' : ' ';
                } else {
                    $value = $name . ': ';
                }

                // let's find phrase (name) and/or addr-spec
                if (preg_match('/^<' . $email_regexp . '>$/', $part)) {
                    $value .= $part;
                } else if (preg_match('/^' . $email_regexp . '$/', $part)) {
                    // address without brackets and without name
                    $value .= $part;
                } else if (preg_match('/<*' . $email_regexp . '>*$/', $part, $matches)) {
                    // address with name (handle name)
                    $address = $matches[0];
                    $word = str_replace($address, '', $part);
                    $word = trim($word);
                    // check if phrase requires quoting
                    if ($word) {
                        // non-ASCII: require encoding
                        if (preg_match('#([\x80-\xFF]){1}#', $word)) {
                            if ($word[0] == '"' && $word[strlen($word)-1] == '"') {
                                // de-quote quoted-string, encoding changes
                                // string to atom
                                $search = array("\\\"", "\\\\");
                                $replace = array("\"", "\\");
                                $word = str_replace($search, $replace, $word);
                                $word = substr($word, 1, -1);
                            }
                            // find length of last line
                            if (($pos = strrpos($value, $eol)) !== false) {
                                $last_len = strlen($value) - $pos;
                            } else {
                                $last_len = strlen($value);
                            }
                            $word = Mail_mimePart::encodeHeaderValue(
                                $word, $charset, $encoding, $last_len, $eol
                            );
                        } else if (($word[0] != '"' || $word[strlen($word)-1] != '"')
                            && preg_match('/[\(\)\<\>\\\.\[\]@,;:"]/', $word)
                        ) {
                            // ASCII: quote string if needed
                            $word = '"'.addcslashes($word, '\\"').'"';
                        }
                    }
                    $value .= $word.' '.$address;
                } else {
                    // addr-spec not found, don't encode (?)
                    $value .= $part;
                }

                // RFC2822 recommends 78 characters limit, use 76 from RFC2047
                $value = wordwrap($value, 76, $eol . ' ');
            }

            // remove header name prefix (there could be EOL too)
            $value = preg_replace(
                '/^'.$name.':('.preg_quote($eol, '/').')* /', '', $value
            );

        } else {
            // Unstructured header
            // non-ASCII: require encoding
            if (preg_match('#([\x80-\xFF]){1}#', $value)) {
                if ($value[0] == '"' && $value[strlen($value)-1] == '"') {
                    // de-quote quoted-string, encoding changes
                    // string to atom
                    $search = array("\\\"", "\\\\");
                    $replace = array("\"", "\\");
                    $value = str_replace($search, $replace, $value);
                    $value = substr($value, 1, -1);
                }
                $value = Mail_mimePart::encodeHeaderValue(
                    $value, $charset, $encoding, strlen($name) + 2, $eol
                );
            } else if (strlen($name.': '.$value) > 78) {
                // ASCII: check if header line isn't too long and use folding
                $value = preg_replace('/\r?\n[\s\t]*/', $eol . ' ', $value);
                $tmp = wordwrap($name.': '.$value, 78, $eol . ' ');
                $value = preg_replace('/^'.$name.':\s*/', '', $tmp);
                // hard limit 998 (RFC2822)
                $value = wordwrap($value, 998, $eol . ' ', true);
            }
        }

        return $value;
    }
function encodeHeader($name, $value, $charset='ISO-8859-1',
        $encoding='quoted-printable', $eol="\r\n"
    ) {
        // Structured headers
        $comma_headers = array(
            'from', 'to', 'cc', 'bcc', 'sender', 'reply-to',
            'resent-from', 'resent-to', 'resent-cc', 'resent-bcc',
            'resent-sender', 'resent-reply-to',
            'mail-reply-to', 'mail-followup-to',
            'return-receipt-to', 'disposition-notification-to',
        );
        $other_headers = array(
            'references', 'in-reply-to', 'message-id', 'resent-message-id',
        );

        $name = strtolower($name);

        if (in_array($name, $comma_headers)) {
            $separator = ',';
        } else if (in_array($name, $other_headers)) {
            $separator = ' ';
        }

        if (!$charset) {
            $charset = 'ISO-8859-1';
        }

        // Structured header (make sure addr-spec inside is not encoded)
        if (!empty($separator)) {
            // Simple e-mail address regexp
            $email_regexp = '([^\s<]+|("[^\r\n"]+"))@\S+';

            $parts = Mail_mimePart::_explodeQuotedString($separator, $value);
            $value = '';

            foreach ($parts as $part) {
                $part = preg_replace('/\r?\n[\s\t]*/', $eol . ' ', $part);
                $part = trim($part);

                if (!$part) {
                    continue;
                }
                if ($value) {
                    $value .= $separator==',' ? $separator.' ' : ' ';
                } else {
                    $value = $name . ': ';
                }

                // let's find phrase (name) and/or addr-spec
                if (preg_match('/^<' . $email_regexp . '>$/', $part)) {
                    $value .= $part;
                } else if (preg_match('/^' . $email_regexp . '$/', $part)) {
                    // address without brackets and without name
                    $value .= $part;
                } else if (preg_match('/<*' . $email_regexp . '>*$/', $part, $matches)) {
                    // address with name (handle name)
                    $address = $matches[0];
                    $word = str_replace($address, '', $part);
                    $word = trim($word);
                    // check if phrase requires quoting
                    if ($word) {
                        // non-ASCII: require encoding
                        if (preg_match('#([\x80-\xFF]){1}#', $word)) {
                            if ($word[0] == '"' && $word[strlen($word)-1] == '"') {
                                // de-quote quoted-string, encoding changes
                                // string to atom
                                $search = array("\\\"", "\\\\");
                                $replace = array("\"", "\\");
                                $word = str_replace($search, $replace, $word);
                                $word = substr($word, 1, -1);
                            }
                            // find length of last line
                            if (($pos = strrpos($value, $eol)) !== false) {
                                $last_len = strlen($value) - $pos;
                            } else {
                                $last_len = strlen($value);
                            }
                            $word = Mail_mimePart::encodeHeaderValue(
                                $word, $charset, $encoding, $last_len, $eol
                            );
                        } else if (($word[0] != '"' || $word[strlen($word)-1] != '"')
                            && preg_match('/[\(\)\<\>\\\.\[\]@,;:"]/', $word)
                        ) {
                            // ASCII: quote string if needed
                            $word = '"'.addcslashes($word, '\\"').'"';
                        }
                    }
                    $value .= $word.' '.$address;
                } else {
                    // addr-spec not found, don't encode (?)
                    $value .= $part;
                }

                // RFC2822 recommends 78 characters limit, use 76 from RFC2047
                $value = wordwrap($value, 76, $eol . ' ');
            }

            // remove header name prefix (there could be EOL too)
            $value = preg_replace(
                '/^'.$name.':('.preg_quote($eol, '/').')* /', '', $value
            );

        } else {
            // Unstructured header
            // non-ASCII: require encoding
            if (preg_match('#([\x80-\xFF]){1}#', $value)) {
                if ($value[0] == '"' && $value[strlen($value)-1] == '"') {
                    // de-quote quoted-string, encoding changes
                    // string to atom
                    $search = array("\\\"", "\\\\");
                    $replace = array("\"", "\\");
                    $value = str_replace($search, $replace, $value);
                    $value = substr($value, 1, -1);
                }
                $value = Mail_mimePart::encodeHeaderValue(
                    $value, $charset, $encoding, strlen($name) + 2, $eol
                );
            } else if (strlen($name.': '.$value) > 78) {
                // ASCII: check if header line isn't too long and use folding
                $value = preg_replace('/\r?\n[\s\t]*/', $eol . ' ', $value);
                $tmp = wordwrap($name.': '.$value, 78, $eol . ' ');
                $value = preg_replace('/^'.$name.':\s*/', '', $tmp);
                // hard limit 998 (RFC2822)
                $value = wordwrap($value, 998, $eol . ' ', true);
            }
        }

        return $value;
    }
Encodes a header as per RFC2047

@param string $name     The header name
@param string $value    The header data to encode
@param string $charset  Character set name
@param string $encoding Encoding name (base64 or quoted-printable)
@param string $eol      End-of-line sequence. Default: "\r\n"

@return string          Encoded header data (without a name)
@access public
@since 1.6.1