PHP更改日期格式(日期来自MySQL's 'CURRENT_TIMESTAMP')

我尝试了一些方法，但到目前为止都没有成功。

我想让yyyy-mm-dd的日期看起来像mm/dd/yyyy

初始值示例，取自MySQL: 2015-08-21 14:46:00

这是我得到的:

foreach ($draftOrders as $key => $value) {
    $timestamp = strtotime($value['last_modified']);
    // this one works (leaving it unchanged)
    // however the slashes are not replacing the dash
    // $new_date_format = date('Y/m/d H:i:s', $timestamp);
    // fails, date becomes 1970-01-01 12:00 am
    $new_date_format = date('m/d/Y H:i:s', $timestamp);
    $draftOrders[$key]['last_modified'] = $new_date_format;
}