PHP通过另一个数组获取动态嵌套数组值

I have a specific issue. I need to get and set values within a nested array. The array differs from occasion to occasion (dynamic) but I have access to another array (let's call it parent array) that contains the path to the nested values.

The array (let's call it main array) with the value that I want to access or change can look like this:

$form = array(
    'field => array(
        'und' => array(
            0 => array(
                'some_key' => 'foo'
            )
        )
    )
);

..and in that case the parent array looks like this:

$parents = array(
    'field',
    'und',
    0,
);

But the main array can also look like this:

$form = array(
    'field => array(
        'und' => array(
            0 => array(
                'another_field' => array(
                    'und' => array(
                        0 => array(
                            'some_key' => 'foo'
                        )
                    )
                )
            )
        )
    )
);

The parent array then looks like this.

$parents = array(
    'field',
    'und',
    0,
    'another field',
    'und'
    0
);

I want to be able to access values that are in a nested array, i.e:

$value = $form['field']['und'][0]['some_key']

And I can't do that since the value I need will be in a different place the next time around. The only thing that informs me of how deep the value is in the nested array is the parent array.

If it was a fixed array I would be able to do like this:

$value = $form[$parent[0]][$parent[1]][$parent[3]]['some_key'];

But it's not..

How do I go about and construct the.. the.. meh, I don't even know what it's called. Path? I want to construct the above $value array based on the information found in the parent array.

I'd like to be able to do something like:

$path = '';
foreach ($parents as $key => $value) {
    $path = $path . '[' . $value . ']';
}
$value = $form . $path . ['some_key'];

Note: For anyone wondering this is Drupals' Form API at work, but the question is more about PHP than Drupal.