code: protected function getValues($keys)
      {
          $results = [];
          foreach ($keys as $key) {
              $results[$key] = $this->getValue($key);
          }
  
          return $results;
      }
docs: Retrieves multiple values from cache with the specified keys.
  The default implementation calls [[getValue()]] multiple times to retrieve
  the cached values one by one. If the underlying cache storage supports multiget,
  this method should be overridden to exploit that feature.
  @param array $keys a list of keys identifying the cached values
  @return array a list of cached values indexed by the keys