MismatchSenderId with Web Push PHP CURL Firebase

I am getting the following error when trying to send a Push Notification using Google Firebase:

{"multicast_id":1559489545169770337,"success":0,"failure":1,"canonical_ids":0,"results":[{"error":"MismatchSenderId"}]}

Can anyone give any clues why please?

It is a new setup and not being imported from GMC.

manifest.json contains:

"gcm_sender_id": "1225****"

This matches "Project Settings" > "Cloud Messaging" > [Sender ID]

The code for registering the user is:

 function urlBase64ToUint8Array(base64String) {
    var padding = '='.repeat((4 - base64String.length % 4) % 4);
    var base64 = (base64String + padding)
        .replace(/\-/g, '+')
        .replace(/_/g, '/');

    var rawData = window.atob(base64);
    var outputArray = new Uint8Array(rawData.length);

    for (var i = 0; i < rawData.length; ++i) {
        outputArray[i] = rawData.charCodeAt(i);
    }
        return outputArray;
    }

    function subscribePush() {
  
  
    navigator.serviceWorker.ready.then(function(registration) {
      if (!registration.pushManager) {
        alert('Your browser doesn\'t support push notification.');
        return false;
      }


    
      registration.pushManager.subscribe({
        userVisibleOnly: true //Set user to see every notification
        , applicationServerKey: urlBase64ToUint8Array('*******') 
        //The "PUBLIC KEY PAIR" under Web configuration. Have tried with and without urlBase64ToUint8Array()
      })
      .then(function (subscription) {
      
        console.info('Push notification subscribed.');
        console.log(subscription);
        //saveSubscriptionID(subscription);
      })
      .catch(function (error) {
        console.error('Push notification subscription error: ', error);
      });
  
 
  
    })
}

My code is registering the user and the response from Firebase is (including "REGISTRATION ID"):

Data {"endpoint":"https://fcm.googleapis.com/fcm/send/****:*****","expirationTime":null,"keys":{"p256dh":"****","auth":"****"}}

I am then using this PHP cURL:

$id = "****:*****"; // "REGISTRATION ID" from the response above. If this is wrong it throws an error ("InvalidRegistration"), so I know that this is correct.


$url = 'https://fcm.googleapis.com/fcm/send';

$fields = array (
        'registration_ids' => array (
                $id
        ),
        'data' => array (
                "message" => "Test"
        )
);
$fields = json_encode ( $fields );

$headers = array (
        'Authorization: key=' . "********", //This is the "Server key" above "Sender ID"
        //This matches "Project Settings" > "Cloud Messaging" > [Server Key]
        //If this is wrong it returns: INVALID_KEY error 401. So I know this is correct.
        'Content-Type: application/json'
);

$ch = curl_init ();
curl_setopt ( $ch, CURLOPT_URL, $url );
curl_setopt ( $ch, CURLOPT_POST, true );
curl_setopt ( $ch, CURLOPT_HTTPHEADER, $headers );
curl_setopt ( $ch, CURLOPT_RETURNTRANSFER, true );
curl_setopt ( $ch, CURLOPT_POSTFIELDS, $fields );

$result = curl_exec ( $ch );
echo $result;
curl_close ( $ch );

So I eventually figured this out. Instead of using the Subscription ID from the end of

https://fcm.googleapis.com/fcm/send/ABCDEF:KJHASKHDASDetc

I used the following code, and used the TOKEN instead and used that as the "registration_ids" in the CURL post

  <script src="https://www.gstatic.com/firebasejs/8.2.6/firebase-app.js"></script>
    <script src="https://www.gstatic.com/firebasejs/8.2.6/firebase-messaging.js"></script>

<!-- TODO: Add SDKs for Firebase products that you want to use
     https://firebase.google.com/docs/web/setup#available-libraries -->
<script src="https://www.gstatic.com/firebasejs/8.2.6/firebase-analytics.js"></script>

<script>
  // Your web app's Firebase configuration
  // For Firebase JS SDK v7.20.0 and later, measurementId is optional
  var firebaseConfig = {
    apiKey: "****",
    authDomain: "****",
    projectId: "****",
    storageBucket: "****",
    messagingSenderId: "****",
    appId: "****",
    measurementId: "****"
  };
  // Initialize Firebase
  firebase.initializeApp(firebaseConfig);
  firebase.analytics();
  
  
  const messaging = firebase.messaging();
  
  messaging.requestPermission()
    .then(function() {
      console.log('Notification permission granted.');
      return messaging.getToken()
    })
    .then(function(result) {
        console.log("The token is: ", result);
    })
    .catch(function(err) {
      console.log('Unable to get permission to notify.', err);
    });

  
  messaging.getToken({ vapidKey: '*****-*****' }).then((currentToken) => {
  if (currentToken) {
    // Send the token to your server and update the UI if necessary
    // ...
    console.log("currentToken: ", currentToken);
  } else {
    // Show permission request UI
    console.log('No registration token available. Request permission to generate one.');
    // ...
  }
}).catch((err) => {
  console.log('An error occurred while retrieving token. ', err);
  // ...
});