I have a full-time job and driving PHV and taxi as well. How do I declare my income earned through PHV and taxi?

You will need to declare your side income to IRAS when filing your income tax.

If your company you are working for your day job files for you, you will need to log in to IRAS portal to update with the side income amount. Very likely you will be asked to contribute to your Medisave CPF account after you receive your notice.

Hope this helps.