在PHP中将MySQL输出格式化为数组

我有一个疑问：

$query = 'SELECT `trades`.`id` AS `trade.id` FROM `trades` WHERE `id` = '. $this->trade->id . ' ORDER BY `created_at` DESC LIMIT 1';

哪个（当然）输出

我如何让它输出这个呢

array(1) {
  ["trade"]=> array(1) {
    ["id"]=> string(2) "44"
  }
}

检查一下。我希望工作

array(1) {
  ["trade"]=> array(1) {
    ["id"]=> string(2) "44"
  }
}

$query = 'SELECT `trades`.`id` AS `id` FROM `trades` WHERE `id` = '. $this->trade->id . ' ORDER BY `created_at` DESC LIMIT 1';
$arr['trade']=$query;
print_r($arr);