Online PHP editor | refs for 3BfKD

<?php $input = [ 'firstName' => null, 'lastName' => null, 'category' => [ 'name' => null, 'service' => [ 'foo' => [ 'bar' => null ] ] ] ]; function prepareFields(array $array): array { return array_reduce(array_keys($array), function ($result, $key) use ($array) { $result[] = $key; if (is_array($array[$key])) { $result[$key] = prepareFields($array[$key]); } return $result; }); } print_r(prepareFields($input));