Is it OK to access one database with two endpoints, eg with PHP in web app and GraphQL in mobile app? Or should I create one API to use for both web and mobile?

Sure, but it would lead to you having to maintain two API’s instead of one, why would you want that?

agreed with @halborg, but there is nothing preventing you do do two APIs if it makes more sense in your use case 🙂

One of the beauties of GQL is that you no more have to deal with

/api/web/someModel/with-details

and 4 other variants of that endpoint to account for different devices and subtypes of that model - you let the clients decide for themselves 🙂 So sure, you could have two API’s but I’d argue that it would be shooting yourself in the foot 🙂