filter value for nested array

filter.php

<?php

$post = [
    'k' => 'v',
    'kk' => [
        'll' => 'あああ',
        'l2' => [
            ['xCode' => 'おおお'],
            ['x2Code' => 'おおお'],
            'm1' => [
                'd1' => '3',
                'd2' => 4,
            ]
        ]
    ]
];

function filter ($key, $value) {
    if (is_array($value)) {
        foreach ($value as $childKey => $childValue) {
            list($childKey, $childValue) = filter($childKey, $childValue);
            $value[$childKey] = $childValue;
        }
        return [$key, $value];
    } else {
        $value = $value . 'fooo'; // filter
        return [$key, $value];
    }
}

foreach ($post as $k => $v) {
    list($k, $v) = filter($k, $v);
    $post[$k] = $v;
}

echo json_encode($post, JSON_PRETTY_PRINT|JSON_UNESCAPED_UNICODE);