how to get title of viewed page

I want check with each page request if the page has changed.

I am using wp_title for it. I am saving the title in a session variable $_SESSION:

$biq_title = wp_title('',false);
  if ( $biq_title == '' )
    $biq_title = 'Home';
  if ( $_SESSION['biq_last_title'] )
  {
    //  check if page has changed
    $biq_last_title = $biq_title;
    if ( $_SESSION['biq_last_title'] == $biq_last_title )
      return false;
    else
    {
      $_SESSION['biq_last_title'] = $biq_title;
      return true;
    }
  }
  else
    $_SESSION['biq_last_title'] = $biq_title;

  return true;

the session variable looses its value and contains a ‘page not found’.

If the session variable looses its value I would expect a NULL value, but how it gets the ‘page not found’????