php是怎么实现事件监听的

class Event
{
 protected static $listens = array();
 
 public static function listen($event, $callback, $once=false){
 if(!is_callable($callback)) return false;
 self::$listens[$event][] = array('callback'=>$callback, 'once'=>$once);
 return true;
 }
 
 public static function one($event, $callback){
 return self::listen($event, $callback, true);
 }
 
 public static function remove($event, $index=null){
 if(is_null($index))
 unset(self::$listens[$event]);
 else
 unset(self::$listens[$event][$index]);
 }
 
 public static function trigger(){
 if(!func_num_args()) return;
 $args = func_get_args();
 $event = array_shift($args);
 if(!isset(self::$listens[$event])) return false;
 foreach((array) self::$listens[$event] as $index=>$listen){
 $callback = $listen['callback'];
 $listen['once'] && self::remove($event, $index);
 call_user_func_array($callback, $args);
 }
 }
}