code: public function setEcoMode($mode, $serial_number = NULL) {
          $serial_number = $this->getDefaultSerial($serial_number);
          $data = array();
          $data['mode'] = $mode;
          $data['touched_by'] = 4;
          $data['mode_update_timestamp'] = time();
          $data = json_encode(array('eco' => $data));
          return $this->doPOST("/v2/put/device." . $serial_number, $data);
      }
docs: Set the thermostat to use ECO mode ($mode = ECO_MODE_MANUAL) or not ($mode = ECO_MODE_SCHEDULE).
  
  @param string $mode          One of the ECO_MODE_* constants.
  @param string $serial_number The thermostat serial number. Defaults to the first device of the account.
  
  @return stdClass|bool The object returned by the API call, or FALSE on error.