* @param int $encodeFlags flags to pass to json encode, defaults to DEFAULT_JSON_FLAGS
* @param bool $ignoreErrors whether to ignore encoding errors or to throw on error, when ignored and the encoding fails, "null" is returned which is valid json for null
* @throws \RuntimeException if encoding fails and errors are not ignored
* @return string when errors are ignored and the encoding fails, "null" is returned which is valid json for null