class My_Plugin {

  private $error;

  public function __construct()
  {
    add_shortcode('my_plugin_form', array($this, 'form_shortcode'));
  }

  public function setError($value)
  {
    $this->error = $value;
  }

  public function getError()
  {
    return $this->error;
  }

  public function form_validation()
  {
    // Validation has failed
    $this->setError('Please complete all of the fields.');
  }

  public function form_shortcode()
  {
    $err = $this->getError();

    ob_start();

    // Some HTML

    $this->getError();

    // Some more HTML

    $html = ob_get_contents();
    ob_end_clean();
    return $html;
  }
}