class Xml

final class Xml {
    
    /**
     * Escapes a string for the use in XML documents.
     *
     * Any Unicode character is allowed, excluding the surrogate blocks, FFFE,
     * and FFFF (not even as character reference).
     *
     * @see https://www.w3.org/TR/xml/#charsets
     */
    public static function prepareString(string $string) : string {
        return preg_replace('/[\\x00-\\x08\\x0b\\x0c\\x0e-\\x1f\\x7f]/', '', htmlspecialchars(self::convertToUtf8($string), ENT_QUOTES));
    }
    private static function convertToUtf8(string $string) : string {
        if (!self::isUtf8($string)) {
            $string = mb_convert_encoding($string, 'UTF-8');
        }
        return $string;
    }
    private static function isUtf8(string $string) : bool {
        $length = strlen($string);
        for ($i = 0; $i < $length; $i++) {
            if (ord($string[$i]) < 0x80) {
                $n = 0;
            }
            elseif ((ord($string[$i]) & 0xe0) === 0xc0) {
                $n = 1;
            }
            elseif ((ord($string[$i]) & 0xf0) === 0xe0) {
                $n = 2;
            }
            elseif ((ord($string[$i]) & 0xf0) === 0xf0) {
                $n = 3;
            }
            else {
                return false;
            }
            for ($j = 0; $j < $n; $j++) {
                if (++$i === $length || (ord($string[$i]) & 0xc0) !== 0x80) {
                    return false;
                }
            }
        }
        return true;
    }

}