如何向应用用户发送通知？

I'm new to android and php, I've been watching tutorials and implementing it. I'm making an app in which i want to send push notifications, I've come across many tutorials but some of them were only implemented on a single device where in my case i want to send it to all my app users(students who will be using my app). If there's any way to deliver messages via notification, I've made an app and in Live section, I want to send push notification inside an activity. How do i do it? I've integrated firebase in my app but it doesn't support multiline messsage's. I'm fairly new to php, If there's any tutorial I could implement please suggest.