Possible problem: you are sending a HTTP header while already having some data in output buffer

V presenteru mám umístěn button, na který když se klikne, tak se vykoná následující kód:

public function handleSaveResponse() {

    $this->db->begin();
    try {

      // Retrieve ID
      $userId = $this->user->id;
      $offerId = $this->id;

      // Load offer
      $offer = $this->createDao("Offer", $offerId);
      if ($offer == null || !$offer->isPopulated()) {
        throw new \Exception('No offer or demand specified for response.');
      }

      // Retrieve advertisement object
      $advert = $offer->getLazyAdvertisement();
      if ($advert == null || !$advert->isPopulated()) {
        throw new \Exception('Cannot load advertisement for offer.');
      }

      // Check active
      if ($advert->getState() !== 'active') {
        throw new \Exception('Cannot response inactive offer.');
      }

      // Check author
      if ($userId === $advert->getAuthorId()) {
        throw new \Exception('Cannot post response for own advertisement.');
      }

      // Select price and save response
      $q = 'SELECT price_total FROM view_offer WHERE offer_id=%i';
      $p = $this->db->fetchSingle($q, intval($offer->getId()));
      if ($p == false) {
        throw new \Exception('Cannot select price of advertised item(s).');
      }
      $this->createDao("Response")->createForOffer($offer->getId(), $userId, $p);
    } catch (\Exception $e) {
      $this->db->rollback();
      throw $e;
    }

    // Commit and redirect to list of responses
    $this->db->commit();
    $this->redirect("Response:list");
  }

Vyvolá se ale tato chyba

Possible problem: you are sending a HTTP header while already having some data in output buffer. Try OutputDebugger or start session earlier.

Poradíte proč?