PHP - long polling for notifications (PHP flags or another way?)

Im starting a new project very soon, and im considering long polling to notify the users that they have a new private message/Notification that they have to check, really similar to how Facebook uses to notify you that someone has posted something about you/Liked a photo of you..

From what I have read, cometd seemed like a really good option to start with.

Then, other ways started emerging, like: Socket.io, and node.js ..

Now, my question is; Which one do you think is the best option for this case and why?

What I need to do exactly is the following;

• User 1 logs into their account
• User 2 sends User 1 a message which gets stored into the database and a flag is generated, (If possible?!)
• The PHP script responsible for User 1 should pick up the flag, and push a notification to User 1

I know how to take care of the javascript side, but I have never done anything similar to long polling. Im using jQuery as javascript library and PHP for the server side.

So recommendations and any good resources to do this?