Stackable 是 Threaded 的一个别称，直到 pthreads v.2.0.0

/**
* Stackable is an alias of Threaded. This class name was used in pthreads until
* version 2.0.0
* @link http://www.php.net/manual/en/class.threaded.php
*/
class Stackable extends Threaded implements Traversable, Countable, ArrayAccess {

}