关于合并二维数组的问题
有一个数组：
array
(
    [0] => array
        (
            [a] => array
                (
                    [id] => 4
                )
)
[1] => array
        (
            [a] => array
                (
                    [id] => 5
                )
)
[2] => array
        (
            [b] => array
                (
                    [id] => 4
                )
)
[3] => array
        (
            [b] => array
                (
                    [id] => 7
                )
)
)
请问如何合并成
array(
[0]=>4
[1]=>5
[2]=>4
[3]=>7
)
value都是二维数组id
------解决思路----------------------
$arr = array
(
'0' => array
(
'a' => array
(
'id' => 4
)
),
'1' => array
(
'a' => array
(
'id' => 5
)
),
'2' => array
(
'b' => array
(
'id' => 4
)
),
'3' => array
(
'b' => array
(
'id' => 7
)
)
);
$new_arr = array();
foreach($arr as $vl){
foreach($vl as $v) $new_arr[] = $v['id'];
}
print_r($new_arr);
------解决思路----------------------
$a = array (
  0 => 
  array (
    'a' => 
    array (
      'id' => '4',
    ),
  ),
  1 => 
  array (