Recursive array filtering

note to self .. a nice clean way to do recursive array filters in PHP

$arr = array(
    'teSt',
    'The quick BroWn ...',
    'foO',
    array(
       'Test sTring'
    )
);

$filter = function(&$input) use (&$filter) {
    if (is_array($input)) $input = array_map($filter, $input);
    else if (is_string($input)) $input = strtolower($input);

    return $input;
};

$filter($arr);
var_dump($arr);