Hello [email address removed],
Thanks for posting in the Community space. At the moment we're unable to add new tax rate in QBO. In QuickBooks Online once we've turned on the GST center, the default tax rate 10% GST will be created by the system for you.
If you needed to create a new tax rate in QuickBooks Online using PHP language, I'd recommend contacting our Intuit Developers through https://developers.intuit.com/app/developer/homepage and click on Contact us and submit a ticket. They'll be the best support that can guide you in creating the tax rate using an API.
To learn more about how to set up the GST and BAS in QuickBooks, please check this link: Set up GST and BAS in QuickBooks Online.
Post again in the Community if you have further concerns.