code: protected function setValue($key, $value, $duration)
      {
          // Use UNIX timestamp since it doesn't have any limitation
          // @see https://secure.php.net/manual/en/memcache.set.php
          // @see https://secure.php.net/manual/en/memcached.expiration.php
          $expire = $duration > 0 ? $duration + time() : 0;
  
          return $this->useMemcached ? $this->_cache->set($key, $value, $expire) : $this->_cache->set($key, $value, 0, $expire);
      }
docs: Stores a value identified by a key in cache.
  This is the implementation of the method declared in the parent class.
  
  @param string $key the key identifying the value to be cached
  @param mixed $value the value to be cached.
  @see [Memcache::set()](https://secure.php.net/manual/en/memcache.set.php)
  @param int $duration the number of seconds in which the cached value will expire. 0 means never expire.
  @return bool true if the value is successfully stored into cache, false otherwise