How to modify external variable from inside recursive function using userdata argument.

<?php
$arr = [
   'one' => ['one_one' => 11, 'one_two' => 12],
   'two' => 2
];

$counter = 0;

//Does not persist
array_walk_recursive( $arr, function($value, $key, $counter) {
   $counter++;
   echo "$value : $counter";  
}, $counter);
echo "counter : $counter"; 

// result
// 11 : 1
// 12 : 1
// 2 : 1
// counter: 0 

//Persists only in same array node
array_walk_recursive( $arr, function($value, $key, &$counter) {
   $counter++;
   echo "$value : $counter";  
}, $counter);

// result
// 11 : 1
// 12 : 2
// 2 : 1
// counter : 0

//Fully persistent. Using 'use' keyword
array_walk_recursive( $arr, function($value, $key) use (&$counter) {
   $counter++;
   echo "$value : $counter";  
}, $counter);
echo "counter : $counter"; 

// result
// 11 : 1
// 12 : 2
// 2 : 3
// counter : 3