code: protected function getFromLookup(/*# string */ $key)
      {
          // found already ? or try find
          if ($key === $this->cache_key || $this->hasInLookup($key)) {
              return $this->getFromRegistry($this->cache_reg, $key);
          }
          // not found
          return null;
      }
docs: get from lookup pool, return NULL if not found
  
  @param  string $key
  @return mixed|null
  @access protected