You can use any backend you want with Flutter, so PHP is fine. AppWrite might be worth taking a look at as it is written in PHP, seems to be in active development and is geared towards being a backend for mobile apps.

Since your backend will likely expose its functionality over HTTP, you can definitely use PHP in combination with Dart/Flutter. You can use whatever as your backend.

Since Flutter is backed by Google, you get lots of great functionality out-of-the box if you are using Firebase as your backend though. You can even get very far without writing any backend-code. I would recommend having a look at that as well.