Pushbullet is a great alternative for this.
However the user needs to have a Pushbullet account and the app installed (iOS, Android) or plugin installed (Chrome, Opera, Firefox and Windows).
You can use the API by creating a Pushbullet app, and connect your application's user to the Pushbullet user using oAuth2.
Using a library would make it much easier, for PHP I could recommend ivkos/Pushbullet-for-PHP.