Creating any kind of game is not as easy as it sounds, let alone a multilayer game. There are several things you can do to create a multiplayer game. The 1st one would be through the use of PHP, MySQL, etc. These kinds are rather slow and would be recommended for games like chess, tictactoe, checkers, etc. The 2nd type would be through the use of sockets. Socket will allow you to create a more real time interaction between players and can allow you to create games that require faster reaction times. In creating a game, you should ask yourself the following:

1.) What kind of game are you making?
2.) Is it feasible in flash and what technique should be used?
3.) Do you have enough experience in using flash to create a game?
4.) Do you have the time to create such a game?
5.) Do you think you'll be interested all the way in making such a game?

Don't overlook any of these questions because game development of any kind is very difficult and time consuming.

Finally, there is no single code that can help you create a good multiplayer game. If you're really willing to make a game then my suggestion is to do some experiments in flash on how to use PHP, MySQL and the XMLSockets to accomplish things. Once you get a good feel on how to use them, you would have completed the 1st step to creating multiplayer games in flash.

--trigger-happy