In PART I: How to implement GCM PhP Push Server for android, I have concluded with a function to send a push notification to a single device.
This part only describes the function that may be used to send to multiple devices the same push message.
Here is a small trick that I use sometimes to show a related content in an activity. In the back of my mind, my intention might be to mimic the iphone's presentModalViewController. This would specifically be useful when you have a list of items which users might repeatedly click items to view an Expanded view of them. I prefer to use this instead of a new Activity.