class Mail

class Mail {
    
    /**
     * RFC-2822 "specials" characters.
     */
    const RFC_2822_SPECIALS = '()<>[]:;@\\,."';
    
    /**
     * Return a RFC-2822 compliant "display-name" component.
     *
     * The "display-name" component is used in mail header "Originator" fields
     * (From, Sender, Reply-to) to give a human-friendly description of the
     * address, i.e. From: My Display Name <xyz@example.org>. RFC-822 and
     * RFC-2822 define its syntax and rules. This method gets as input a string
     * to be used as "display-name" and formats it to be RFC compliant.
     *
     * @param string $string
     *   A string to be used as "display-name".
     *
     * @return string
     *   A RFC compliant version of the string, ready to be used as
     *   "display-name" in mail originator header fields.
     */
    public static function formatDisplayName($string) {
        // Make sure we don't process html-encoded characters. They may create
        // unneeded trouble if left encoded, besides they will be correctly
        // processed if decoded.
        $string = Html::decodeEntities($string);
        // If string contains non-ASCII characters it must be (short) encoded
        // according to RFC-2047. The output of a "B" (Base64) encoded-word is
        // always safe to be used as display-name.
        $safe_display_name = Unicode::mimeHeaderEncode($string, TRUE);
        // Encoded-words are always safe to be used as display-name because don't
        // contain any RFC 2822 "specials" characters. However
        // Unicode::mimeHeaderEncode() encodes a string only if it contains any
        // non-ASCII characters, and leaves its value untouched (un-encoded) if
        // ASCII only. For this reason in order to produce a valid display-name we
        // still need to make sure there are no "specials" characters left.
        if (preg_match('/[' . preg_quote(Mail::RFC_2822_SPECIALS) . ']/', $safe_display_name)) {
            // If string is already quoted, it may or may not be escaped properly, so
            // don't trust it and reset.
            if (preg_match('/^"(.+)"$/', $safe_display_name, $matches)) {
                $safe_display_name = str_replace([
                    '\\\\',
                    '\\"',
                ], [
                    '\\',
                    '"',
                ], $matches[1]);
            }
            // Transform the string in a RFC-2822 "quoted-string" by wrapping it in
            // double-quotes. Also make sure '"' and '\' occurrences are escaped.
            $safe_display_name = '"' . str_replace([
                '\\',
                '"',
            ], [
                '\\\\',
                '\\"',
            ], $safe_display_name) . '"';
        }
        return $safe_display_name;
    }

}