Symfony\Component\HttpFoundation\Response::setEtag PHP Method

    public function setEtag($etag = null, $weak = false)
    {
        if (null === $etag) {
            $this->headers->remove('Etag');
        } else {
            if (0 !== strpos($etag, '"')) {
                $etag = '"'.$etag.'"';
            }

            $this->headers->set('ETag', (true === $weak ? 'W/' : '').$etag);
        }

        return $this;
    }

 /**
  * {@inheritdoc}
  */
 public function update(Response $response)
 {
     // if we have no embedded Response, do nothing
     if (0 === $this->embeddedResponses) {
         return;
     }
     // Remove validation related headers in order to avoid browsers using
     // their own cache, because some of the response content comes from
     // at least one embedded response (which likely has a different caching strategy).
     if ($response->isValidateable()) {
         $response->setEtag(null);
         $response->setLastModified(null);
         $this->cacheable = false;
     }
     if (!$this->cacheable) {
         $response->headers->set('Cache-Control', 'no-cache, must-revalidate');
         return;
     }
     $this->ttls[] = $response->getTtl();
     $this->maxAges[] = $response->getMaxAge();
     if (null !== ($maxAge = min($this->maxAges))) {
         $response->setSharedMaxAge($maxAge);
         $response->headers->set('Age', $maxAge - min($this->ttls));
     }
     $response->setMaxAge(0);
 }