ActionScript 3.0 :: Making An Online Game

I want to make a multiplayer flash game- something very basic, like each person controls a dot which they can move around (doing this as a learning experience, not because it seems like a great concept for a game ). I'm pretty new to PHP and mySQL, and don't have a lot of experience in making flash apps that interact with outside scripts... and here's the approach i've thought up so far.i design the swf so that every 30th of a second or so it checks an xml file on the server (http:servernamedata.xml), and also sends a request to the server to run a php script and sends it four boolean values (which keys the player is pressing). The php script runs- it loads a mysql database which has each players x/y coordinates, adjusts those coordinates according to the bool (if player is pressing keys), resends the x/y coordinates to the mysql database, and generates an updated version of the xml, which the swf running on each players client computer will check to determine where to draw each dot.So to get to the point, what i want to know is:

1: is what i described even possible?

2: if it can be done, is this a good way to go about it or is it horribly innefficient (i eventually want to make games more complicated than moving dots around, and have as much code run server-side as possible to prevent hacking/cheating)

3: if this is a possible and OK way to do this, what sort of AS3 script do you use to make a flash app have a server execute a php script?