yii\httpclient\Message::addData PHP Method

    public function addData($data)
    {
        if (empty($this->_data)) {
            $this->_data = $data;
        } else {
            $this->_data = array_merge($this->_data, $data);
        }
        return $this;
    }

 /**
  * @inheritdoc
  */
 public function addData($data)
 {
     if ($this->isPrepared) {
         $this->setContent(null);
         $this->isPrepared = false;
     }
     return parent::addData($data);
 }