code: protected function buildSignatureKey()
      {
          $signatures = [
              'a' => $this->appId,
              'b' => $this->bucket,
              'k' => $this->secretId,
              'e' => time() + 2592000,
              't' => time(),
              'r' => rand(),
              'u' => '0',
              'f' => ''
          ];
  
          return http_build_query($signatures);
      }
docs: Build signature Key
  
  @return string