php中将二维数组转换成一维数组

//将二维数组转换成为一维数组
//如下为二维数组
Array
(
[0] => Array
(
[card_id] => 24
)

[1] => Array
(
[card_id] => 23
)

[2] => Array
(
[card_id] => 22
)

[3] => Array
(
[card_id] => 21
)

)

//转换成为
array(4) {
[0]=>
int(24)
[1]=>
int(23)
[2]=>
int(22)
[3]=>
int(21)
}
//方法:
$re=array();
foreach ($result1 as $key=>$value) {

$re[$key] = $value[‘card_id’];