关于合并二维数组的有关问题

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 (