Please notice how easy is to introduce an option handling just for ISO-8859-1

    /**
     * JSON encode a string value by escaping characters as necessary
     *
     * @param $value string
     * @return string
     */
    protected function _encodeString(&$string)
    {
        // Escape these characters with a backslash:
        // " \ / \n \r \t \b \f
        $search  = array('\\', "\n", "\t", "\r", "\b", "\f", '"', '/');
        $replace = array('\\\\', '\\n', '\\t', '\\r', '\\b', '\\f', '\"', '\\/');
        $string  = str_replace($search, $replace, $string);

        // Escape certain ASCII characters:
        // 0x08 => \b
        // 0x0c => \f
        $string = str_replace(array(chr(0x08), chr(0x0C)), array('\b', '\f'), $string);
+
+       //Decode string
+       if ('ISO-8859-1' == $this->_options['encoding'])
+               $string = utf8_encode($string);
+
        $string = self::encodeUnicodeString($string);

        return '"' . $string . '"';
    }