[0] => array(
    'id' => 1,
    'parent_id' => 0,
),
[1] => array(
    'id' => 2,
    'parent_id' => 0,
),
[2] => array(
    'id' => 3,
    'parent_id' => 0,
),
[3] => array(
    'id' => 4,
    'parent_id' => 2
),
[4] => array(
    'id' => 5,
    'parent_id' => 1
),
[5] => array(
    'id' => 6,
    'parent_id' => 0
),
[6] => array(
    'id' => 7,
    'parent_id' => 3
)

to:

[0] => array(
    'id' => 1,
    'parent_id' => 0,
),
[4] => array(
    'id' => 5,
    'parent_id' => 1
),
[1] => array(
    'id' => 2,
    'parent_id' => 0,
),
[3] => array(
    'id' => 4,
    'parent_id' => 2
),
[2] => array(
    'id' => 3,
    'parent_id' => 0,
),
[6] => array(
    'id' => 7,
    'parent_id' => 3
)
[5] => array(
    'id' => 6,
    'parent_id' => 0
),