This FCM web app should be run on a remote PHP web server or a local PHP web server by placing the html and php source files in your web root directory.
If this runs on a remote PHP web server, the FCM web app should be accessed trough the https protocol.
To initialize the FCM web app, go to Setup page, and specify the required credentials which can be taken from the Firebase console.
The VAPID key can be taken from the Web Push certificates from the Firebase console.
The Client Email and Private Key which can be taken from the Service Account and are required for authentication for sending the message and subscribing to the topic.
Click the Initialize button to initialize the FCM web app.
After the instance token was created, use this instance token for sending the message from the FCM backend.