I have tried reading some articles, but I am not very clear on the concepts yet.
Would someone like to take a shot at explaining to me what these technologies are:
In the examples below the client is the browser and the server is the webserver hosting the website.
Before you can understand these technologies, you have to understand classic HTTP web traffic first.
The server sends an event to the client when there's new information available.
The server and the client can now send each other messages when new data (on either side) is available.
Now, which one of them should I use for a realtime app (that I need to code). I have been hearing a lot about websockets (with socket.io [a node.js library]) but why not PHP ?
You can use PHP with WebSockets, check out Ratchet.