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 {}