Can I give you some advice sir? When you need help with coding, it would be a good thing to post the whole code, or that part of the code that you need help with. Because that would save you, and those that are helping you, a lot of time by eleminating the guess work.
Example_Controller_Public_Index
{
public function actionExample()
{
$this->_input->filterSingle('example', XenForo_Input::STRING);
//I have come to check an empty field or not?
}
}