Sanya to Phuket via Guangzhou (1.5hr) transit on Southern China Airlines

You do not need a visa. Simply follow the signs of international connection after you arrive just as you would any other airport.

How are you entering Sanya? More information is necessary to give advise. If you entered Sanya with a visa then you can leave through any Chinese City with that same visa. If you are traveling through both Sanya and Guangzhou you are only eligible for 24 hours in all of China to complete your transit

Sorry, did not see you will be going to Sanya. In that case you will need a visa if transiting in another Mainland Chinese city. Only direct arrivals from outside Mainland China do not require a visa in advance (but you may need to wait quite a while before immigration for tourist agencies to get approval for you).