I'm making this as short as I can.
What I've done so far:
The game I have will be running with JavaScript using setTimeout or setInterval. It's not flash or anything special. What I have made so far as like a test run (so you can understand better), is pretty much loop Ajax to keep sending requests to a PHP page as fast as it can, which then PHP reads the $_GET in the url from the request, then PHP edits a file called p1.html with the $_GET, which is simply player 1's x and y-axis coordinates. So in player 2's browser, it pretty much did what I said above, and now when it receives the Ajax request, it will receive the coordinates of player 1. So JavaScript does what it does, and moves player 1 on player 2's browser. That's what I already made myself, and I tested it and yes it works and yes it lasts forever when I fixed all the bugs and stuff.
Saying that, that's what I've made so far, but this isn't fast enough if I wanted like a online fighting game or a real time side scrolling game. So that's what I need help with. I know a lot of stuff on w3schools.com, but I just don't know how to get this job done. That's probably it. I just need browser 1, to get data to browser 2, something like this "1,100,200" or a little longer actually, and have browser 2 read that data as a variable in JavaScript (anything like x="received data";), and that's just it. JavaScript will do the rest.
I'm sure I can program everything I need myself, but for sending data from point A to point B like 50 times a second, I just don't know anything about it. Not even a name. And last, urls and examples would be very nice (to be clearer). Especially if it's already on w3schools.com (with examples) and I just missed it.
The answer is simple : Use websockets. They allow the immediate push of data in both directions for seemingly instantaneous interactions. They eliminate the need of pulling data from the client which is slow and heavy (including for the server). Note that's the solution used in my favorite game for notification push.
You might be interested by this introduction on websockets on PHP. But note that PHP really isn't the most efficient solution for long connections handling and real time games (you might prefer Go, node.js, java, etc.).