Simple Push Notification Android App

We need a basic Android App that will be installed on our staff team phones (less than 20 users). The app's sole purpose is to receive notifications from our PHP admin engine when certain transactions are made, we are currently using emails and SMS to notify the users but would like to implement push notifications to replace that. The app should authenticate the user by querying our existing user table in MySQL DB by submitting user name and password from the app. The notifications must be sent from our servers using PHP. You are only expected to create the PHP code to send a notification, integrating sending the notifications within our system transactions is not in this project's scope. You can use Twilio Notify service or FCM, whichever you prefer. This is the just the beginning of developing our app, we are looking for a long term relationship since we are planning to add further features later.