code: private function setBody($body)
      {
          if (is_null($body)) {
              throw new InvalidArgumentException('Body of message cannot be null');
          }
  
          if (!is_string($body)) {
              throw new InvalidArgumentException(
                  sprintf(
                      'Body of message must be a string, but "%s" given',
                      is_object($body) ? get_class($body) : gettype($body)
                  )
              );
          }
  
          if (mb_strlen($body, 'UTF-8') === 0) {
              throw new InvalidArgumentException('Body of message cannot be an empty string');
          }
  
          $this->body = (string) $body;
      }
docs: /*
  @param string $body