code: public static function encodeJson($native)
      {
          if (function_exists('json_decode') && function_exists('json_last_error')) {
              $json = json_encode($native);
              $error = json_last_error();
              if ($error !== JSON_ERROR_NONE) {
                  $message = function_exists('json_last_error_msg') ? json_last_error_msg() : 'error code: ' . $error;
                  throw new RuntimeException($message, $error);
              }
              return $json;
          }
  
          $encoder = new Services_JSON();
          $json = $encoder->encode($native);
          if (Services_JSON::isError($json)) {
              throw new RuntimeException($json->toString(), $json->getCode());
          }
  
          return $json;
      }
docs: 値を JSON 形式の文字列に変換します。
  
  @param mixed $native
  @throws RuntimeException
  @return string