Make a recursion on array to find if children of parent has also children [PHP]

Question

Ex:

[
[1] => [
  [2],
  [3],
  [4]
],
[4] => [
  [5],
  [6],
  [7]
]
]

how do i loop through the array and find the child(ren) that needs to be attached to they parent child?
the 4 parent needs to be attach to 4 child inside 1.

Answer 1

If any of children of a parent has also children?

$input = [
    1 => [2, 3, 4],
    4 => [5, 6],
    2 => [4, 6]
];
$allparents = array_keys($input);
foreach ($allparents as $parent) {
    foreach ($input as $nodes) {
        if (in_array($parent, $nodes)) {
            $out[] = $parent;
        }
    }
}
$out = array_unique($out);
print_r($out);

will print:

Array
(
    [0] => 4
    [2] => 2
)