How does facebook, gmail send the real time notification?

I have read some posts about this topic and the answers are comet, reverse ajax, http streaming, server push, etc.

How does incoming mail notification on Gmail works?

How is GMail Chat able to make AJAX requests without client interaction?

I would like to know if there are any code references that I can follow to write a very simple example. Many posts or websites just talk about the technology. It is hard to find a complete sample code. Also, it seems many methods can be used to implement the comet, e.g. Hidden IFrame, XMLHttpRequest. In my opinion, using XMLHttpRequest is a better choice. What do you think of the pros and cons of different methods? Which one does Gmail use?

I know it needs to do it both in server side and client side.
Is there any PHP and Javascript sample code?

How does facebook, gmail send the real time notification?