code: public function clear($uid = null) {
          $options = $this->getOptions();
          if ($uid) {
              if (!$this->has($uid)) {
                  return false;
              }
              $stmt = $this->adapter->query(sprintf('DELETE FROM %s WHERE %s = "%s"', $options['table'], $options['column_key'], $uid), Adapter::QUERY_MODE_EXECUTE);
              return true;
          }
          $stmt = $this->adapter->query(sprintf('TRUNCATE TABLE %s', $options['table']), Adapter::QUERY_MODE_EXECUTE);
          return true;
      }
docs: Clear datas with $uid key
  @param mixed $uid
  @return void