How to mutate this array within `array_reduce` - php

Question

Is there a way to mutate an array using array_reduce in PHP?
I'm trying to do something like this:
Given some ordered list of ids:
$array = [["id" => 1], ["id" => 13], ["id" => 4]];
And a tree that has a subtree matching the corresponding ids:
$tree = [
"id" => 2334,
"children" => [
[
"id" => 111,
"children" => []
],
[
"id" => 1, // <- this is a match
"children" => [
[
"id" => 13, // <- this is a match
"children" => [
[
"id" => 4, // <- this is a match
"children" => []
],
[
"id" => 225893,
"children" => []
],
[
"id" => 225902,
"children" => []
]
]
]
]
]
]
];
How can I mutate the arrays in that subtree?
I'm currently trying to use array_reduce to walk down the tree and mutate it. However, the mutation isn't being applied to the originally passed in $tree.
array_reduce($array, function (&$acc, $item) {
$index = array_search($item['id'], array_column($acc['children'], 'id'));
$acc['children'][$index]['mutated'] = true; // mutation here
return $acc['children'][$index];
}, $tree);
echo "<pre>";
var_dump($tree); // $tree is unchanged here
echo "</pre>";
Why is $tree not mutated after the running above array_reduce?
Is there a way to use foreach in this case?

Answer 1

I think this function will do what you want. It recurses down $tree, looking for id values that are in $array and setting the mutation flag for those children:
function mutate(&$tree, $array) {
if (in_array($tree['id'], array_column($array, 'id'))) {
$tree['mutated'] = true;
}
foreach ($tree['children'] as &$child) {
mutate($child, $array);
}
}
mutate($tree, $array);
var_export($tree);
Output:
array (
'id' => 2334,
'children' => array (
0 => array (
'id' => 111,
'children' => array ( ),
),
1 => array (
'id' => 1,
'children' => array (
0 => array (
'id' => 13,
'children' => array (
0 => array (
'id' => 4,
'children' => array ( ),
'mutated' => true,
),
1 => array (
'id' => 225893,
'children' => array ( ),
),
2 => array (
'id' => 225902,
'children' => array ( ),
),
),
'mutated' => true,
),
),
'mutated' => true,
),
),
)
Demo on 3v4l.org