which one is better in phalcon ? using namespace or do not use a namespace ?
Phalcon uses namespaces itself, default path routing is built on namespaces. Also probably you will use many libraries with Phalcon so namespaces will be present in your project anyway. Please see more here: https://docs.phalcon.io/en/latest/reference/namespaces.html
Namespaces all the way. Even in Micro (API) if you use namespaces, it will make your life much easier for the future maintenance.