Well PHP and Mysql is only half the battle, the other part you would have to learn would be flash (when i say that i mean actionscript or flex as well).
We had a similar post in the web game are of the forums, where someone wanted to know how to make a game like XXXXX.
If you want to know how to design a game, then you cant really... its either something you can do or something you cant. Being able to program and do artwork helps, but if you dont know how to work out getting from your start point of having nothing to your end point of having a game with lots of people you will just end up wasting your time.
If you want to know how to implement a game, then you need to learn server side technologies (php/asp/cold fusion... any other random languages) as well as client side technologies (silverlight/flash/javascript/html/css). Then once you know enough about them you will have to learn more about databases (mysql/mssql... other DB engines).
At this point you are ALMOST ready to start implementing your game. You are still missing artworkers and other resources that will acctually provide the content for the game you code, now getting them to work for free on a project that you havent even started yet will be tough. Even if you do get them, there is no telling if they can do the job or not, so you may end up wasting lots of time with crappy art before you get some good ones.
THEN once you have acctually coded your back/front end and got them all communicating and you have had a quick play and it looks nice and functional. You will need to get a server online which may cost between £50-£200 per year to host, and that will only really support a few hundered people if you have coded it to a semi reasonable level.
Now you have to get players, now that will take some advertising as well as alot of work in your communities making sure players are happy and the game is performing how YOU want as well as how the PLAYERS want.
Lets hope you have the time and motivation to follow it through, i didnt even include the part about managing your team and resources
