code: protected function generateEtag($seed)
      {
          $etag = '"' . rtrim(base64_encode(sha1($seed, true)), '=') . '"';
          return $this->weakEtag ? 'W/' . $etag : $etag;
      }
docs: Generates an ETag from the given seed string.
  @param string $seed Seed for the ETag
  @return string the generated ETag