有效地循环遍历数组

Question

I have an array like this:

[1] => Array
        (
            [id] => 7
            [ticket_id] => 12
            [client_id] => 174
            [thread_name] => 
            [added_by] => 2
            [created_at] => 2015-07-28 23:24:07
            [updated_at] => 2015-07-28 23:24:07
            [notes] => Array
                (
                    [0] => Array
                        (
                            [id] => 21
                            [user_id] => 2
                            [notes] => Not all those who wander are lost!
                            [notes_attachment] => 
                            [deleted_at] => 
                            [created_at] => 2015-07-28 23:34:31
                            [updated_at] => 2015-07-28 23:34:31
                            [thread_id] => 7
                            [users] => Array
                                (
                                    [0] => Array
                                        (
                                            [user_id] => 1
                                        )

                                    [1] => Array
                                        (
                                            [user_id] => 2
                                        )

                                )

                        )

                )

        )

And I need to manipulate the users array in the notes array to something like this.

[users] => Array
(
[0] => 1
[1] => 2
)

So, basically, I need an array for ids in the users array.

I tried doing it, but it takes three loop to get into users.

How can I do that more efficiently ??

NOTE: This is just one of the structure from multiple arrays. Like, I have 4 threads and multiple notes in each of them.

Also, I need to manipulate the existing array and not just take the data from it.

My attempt:

$tmp = array();
        foreach($data as $value) {
            foreach($value['notes'] as $notes) {
                if(!empty($notes['users'])){
                    foreach($notes['users'] as $users) {
                        $tmp[] = $users['user_id'];
                        unset($notes['users']);
                        $notes['users'] = $tmp;
                    }
                }
            }
        }

Answer 1

Use array_column -

$new = array_column($a[notes][0]['users'], 'user_id');
var_dump($new);

Output

array(2) {
  [0]=>
  int(1)
  [1]=>
  int(2)
}

DEMO

Update

$new = array();
foreach($a as $value) {
    $new = array_merge($new, array_column($value[notes][0]['users'], 'user_id'));
}
var_dump($new);