The preferred approach on Android is Google Cloud Messaging.
Google Cloud Messaging for Android (GCM) is a service that allows you
to send data from your server to your users' Android-powered device.
This could be a lightweight message telling your app there is new data
to be fetched from the server (for instance, a movie uploaded by a
friend), or it could be a message containing up to 4kb of payload data
(so apps like instant messaging can consume the message directly).
The GCM service handles all aspects of queueing of messages and
delivery to the target Android application running on the target
device. GCM is completely free no matter how big your messaging needs
are, and there are no quotas.
Essentially, it works like this: Your server talks to Google, and Google pushes the message in real-time (or nearly so, as long as the device is powered on and connected to the Internet) to the Android device(s). There's a registration process that happens on the Android device the first time the app is installed (and at future points to revalidate the registration). In your Android app, it's your responsibility to send this registration ID to your server so you can store it for future use. Your server uses this registration ID when it wants to push a message to that device.
The communication protocol between your server and Google is JSON over HTTPS. Your server can use any languages/technologies, as long as it can communicate with Google's servers. A PHP/MySQL application can definitely meet the basic requirements for the service:
Before you can write client Android applications that use the GCM
feature, you must have an application server that meets the following
criteria:
- Able to communicate with your client.
- Able to fire off HTTPS requests to the GCM server.
- Able to handle requests and resend then as needed, using exponential back-off.
- Able to store the API key and client registration IDs.
On the client side, the Android device must be running API version 8 (Android 2.2) or later. Of course, it's also your Android app's responsibility to process incoming messages (using a BroadcastReceiver, as the Android system sends an Intent to your receiver whenever a message arrives), which allows you to handle messages however you want.
