Webview app (IOS, Android) with Firebase notifications.

I just needed to create a webview app for iOS and Android. When the user opens the app, the request is made to send notifications, if the user rejects the notifications a message is shown that he really needs to activate the notifications or the app will not work. When the user accepts, the device is added to an SQL database with the device ID and a category id (ie: Friends). Firebase libraries must be ready to be uploaded to the server.
Then I just need a simple php code to send a notification to all users in the Friends category. I'm a programmer, but I don't understand anything about notifications, so I need this initial help.