How to validate firebase login with facebook

I have the following code executing when a user logs in to facebook:

FB.Event.subscribe('auth.authResponseChange', checkLoginState); 

Here is the code to analayse:

    function checkLoginState(response) {         if (response.authResponse) {             // User is signed-in Facebook.             const unsubscribe = onAuthStateChanged(auth, (firebaseUser) => {                 unsubscribe();                 // Check if we are already signed-in Firebase with the correct user.                 if (!isUserEqual(response.authResponse, firebaseUser)) {                     // Build Firebase credential with the Facebook auth token.                     const credential = FacebookAuthProvider.credential(                         response.authResponse.accessToken);                     // Sign in with the credential from the Facebook user.                     let x = signInWithCredential(auth, credential)                         .catch((error) => {                             // Handle Errors here.                             const errorCode = error.code;                             const errorMessage = error.message;                             // The email of the user's account used.                             const email = error.customData.email;                             // The AuthCredential type that was used.                             const credential = FacebookAuthProvider.credentialFromError(error);                             alert("Login failed. Please try again.");                         });                     x.then((userCredential) => {                         // Signed in                         const user = userCredential.user;                         // login(response.authResponse.accessToken, firebasetoken???);                     });                 } else {                     // User is already signed-in Firebase with the correct user.                     console.log(response);                     // login(response.authResponse.accessToken, firebasetoken???);                 }             });         } else {             // User is signed-out of Facebook.             signOut(auth);         }     } 

I'm unsure how to pass the FIREBASE login token to verify in the backend (with kreait):

        $auth = (new Factory)             ->withServiceAccount($_ENV["PATH"].$_ENV['config']['storage']['firebase']['firebase']['file'] ?? 'firebase-service-account.json')             ->createAuth();         // verify token         $verifiedIdToken = $auth->verifyIdToken($token);         $uid = $verifiedIdToken->getClaim('sub'); // throws an InvalidToken when invalid 

Kreait docs: https://github.com/kreait/firebase-php

Any help is appreciated.