code: protected function setValue($key, $value, $duration)
      {
          return $this->useApcu ? apcu_store($key, $value, $duration) : apc_store($key, $value, $duration);
      }
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. Most often it's a string. If you have disabled [[serializer]],
  it could be something else.
  @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.