code: protected function getCacheFile($key)
      {
          if ($this->directoryLevel > 0) {
              $base = $this->cachePath;
              for ($i = 0; $i < $this->directoryLevel; ++$i) {
                  if (($prefix = substr($key, $i + $i, 2)) !== false) {
                      $base .= DIRECTORY_SEPARATOR . $prefix;
                  }
              }
  
              return $base . DIRECTORY_SEPARATOR . $key . $this->cacheFileSuffix;
          }
  
          return $this->cachePath . DIRECTORY_SEPARATOR . $key . $this->cacheFileSuffix;
      }
docs: Returns the cache file path given the cache key.
  @param string $key cache key
  @return string the cache file path