为啥在 array_walk() 函数里定义的数组，无法在外部打印？

$res = array(); // 在外部定义$res
                    array_walk($data, function (&$v) use ($param, &$res) {
                        $v = json_decode($v, true);
                        if ( $v['appointment_date'] == $param['dateline'] ){
                            $res['venue_nameinfo_id'] = $v['venue_nameinfo_id'];
                            $res['appointment_date'] = $v['appointment_date'];
                            $res['start_time'] = $v['start_time'];
                            $res['end_time'] = $v['end_time'];
                            $res['venueinfo_type'] = $v['venueinfo_type'];
                            $res['venue_nameinfo_title'] = $v['venue_nameinfo_title'];
                            $res['venue_nameinfo_multiplayer'] = $v['venue_nameinfo_multiplayer'];
                        }
                    });
                    var_dump($res);

打印出来的结果是：
array(0) {
}