Using reactJS, php and mysql is possible?

I want to learn ReactJS, is it dependent on a certain backend? No it does not, it depends on your project needs and preference

Is it possible to use ReactJS as frontend, then PHP and MySQL for the backend?

Yes of course, you can build a backend with PHP using a framework like CodeIgniter or Laravel or vanilla PHP and build your API and serve your React App with the data, or you can use Node.js for the backend or Java spring boot or .NET or any backend language just build a good REST API and use the language that fits your project, there is a lot more to say.

Absolutely! The best approach is to write an API in PHP to pass the MySQL data to javascript via REST endpoints. Like this:

https://code.tutsplus.com/tutorials/how-to-build-a-simple-rest-api-in-php--cms-37000

Then use HTML and Javascript to build a react frontend that would access this data. You'd probably want to use a built tool like webpack or even a framework like next.js to compile your javascript and do nice things like code splitting and tree-shaking.

In the above case, they would be two separate code bases. One for the API and one for the frontend. But react is actually very adaptable and you can run any react component on any page very easily using ReactDOM here is an intro:

https://reactjs.org/docs/add-react-to-a-website.html

with this you can put a react component on any existing php website and either build out specific endpoints to populate data or even just fill a <script> tag with JSON and read it with useEffect or componentDidMount