PHP 里面的数组按照某个时间字段排序

$store_list = Array
(
    [0] => Array
        (
            [name] => asdf
            [head] => /upload/thumbs/20150814220142_27630.jpg
            [img] => 
            [content] => asdfasdfasdf
            [datetime] => 2015-10-11 09:55:08
            [flag] => false
        )

    [1] => Array
        (
            [name] => 基本原则顶替
            [head] => 
            [img] => 
            [content] => 基本原则顶替
            [datetime] => 2015-09-01 10:41:08
            [flag] => true
        )

    [2] => Array
        (
            [name] => ad
            [head] => /upload/thumbs/20150814220142_27630.jpg
            [img] => 
            [content] => asdfasdf
            [datetime] => 2015-09-01 09:51:08
            [flag] => false
        )

)

foreach($sort_list as $key=>$v){
$sort_list[$key]['datetime'] = date("YmdHis",strtotime($v['datetime']));
}
$datetime = array();

foreach ($sort_list as $user) {
$datetime[] = $user['datetime'];
}
array_multisort($datetime,SORT_DESC,$sort_list);

print_r()

需要按照数组中的时间进行升序排列。