private function walk(callable $cb)
    {
        foreach($this->values() as $key => $value) {
            $cb($key, $value);
        }
    }
private function walk(callable $cb)
    {
        foreach($this->values() as $key => $value) {
            $cb($key, $value);
        }
    }
private function walk ( callable $ cb ) { foreach ( $ this -> values ( ) as $ key => $ value ) { $ cb ( $ key , $ value ) ; } }
Apply a callback to each input value
Apply a callback to each input value