I'm making a web app for a client who is probably going to use it for commercial use. I'm not affialited with me, nor we are commercial partners.

However, although I'm making this project for this client, I want to be sure I can reuse the same code on future projects.

I know there are many licenses (GPL etc.) but it's not clear to me which one complies the following requirments:

* The code can be sold by me to the client (so he pays me for the code)
* The client may use the code for commercial use
* I still own the ownership on the code, i.e. , I can reuse the code for future works, either in open source works or other projects (which I may sell)

Furthermore, it's not clear to me: When I choose I license how can I "apply" it? I mean: who "certifies" my app is licensed udner that specific licence and do I have to pay for this "certification"?

Thanks in advance :)