text browser game

I'm pretty noobish at web coding / webdev.  I'm trying to figure out how I could write a browser game using only JavaScript on the client-side (ie. no browser plug-ins).  But I'm stuck on figuring out how to handle everything server-side.

The only thing I've thought of is to store everything in a SQL database, and just have the AJAX post some information to a PHP file that will take care of everything.  However, I'm wondering if there are other, better ways of doing this that I can't think of.  I'm also not sure about handling concurrent players because I haven't done very much with databases before, so I'm not sure whether I'll have to worry about race conditions or something.

Any ideas or advice would be appreciated.

It's not much different than any other kind of client-server game, so just define the requests, write the client, write the server which maintains the internal world state, maybe persist it somehow (you mention MySQL, but any kind of persistence can be used). The server-side language can be anything as well, why choose PHP?