phpによるmongodbパケットソート

$keys = array('type'=>1); //  type    
$initial = array("items" => array('CreateTime'=>0)); //  reduce    
$reduce = "function (obj, prev) { 
    if(obj.CreateTime > prev.items.CreateTime){  //  CreateTime   
        prev.items = obj;
    }
    	    
}";

$cand = array('condition'=>array('status'=>1)); //    

$collection->group($keys, $initial, $reduce, $cond);