Firebase-Cloud-Messaging
Firebase Cloud Messaging (FCM)
Firebase Cloud Messaging (FCM) is a cross-platform messaging solution that lets you reliably deliver messages and notifications at no cost.
Using FCM, you can notify a client app that new email or other data is available to sync. You can send notifications to drive user reengagement and retention. For use cases such as instant messaging, a message can transfer a payload of up to 4KB to a client app.
For more information go to http://androidlearning.in/android-push-notifications-using-firebase-cloud-messaging-fcm-php-mysql/