How to exchange real-time updates on a website between users on different computers?

I am trying to find a way for website users (say, student and teacher) to share a secure connection in which real-time viewing on one page is viewed by both of them. From the research, I came to the conclusion that some real-time updates can be performed using ajax and javascript.

But I don’t understand how users can use the connection, where only two users will see updates that occur on the website (for example, flash animation of the drawing board.) I am also confused about how you would even start to establish a connection like this .

I looked at intp php sessions and cookies, but I'm not sure I'm doing the right research. Any guidance on how two specific users can share a secure connection, where real-time viewing is only viewed by both of them. I do not want a short answer, please. I am looking for specific details such as php specific functions and syntax. I appreciate the help and will appreciate you if you give me good answers!