private function checkResponse ($string) {
    if (substr($string, 0, 3) !== '+OK') {
      $this->error = array(
        'error' => "Server reported an error: $string",
        'errno' => 0,
        'errstr' => ''
      );

      if ($this->do_debug >= 1) {
        $this->displayErrors();
      }

      return false;
    } else {
      return true;
    }

  }