Well there are two main components to this problem 1. Getting Php to read/write to a flat file (or db). 2. Getting flash to read it.
The first will be the harder of the two. A quick search in the server side section or on google is sure to get some hits.
The second, is easier, You just have to get your php to output in a way that flash will be able to read it. And once in flash get flash to read it. You will need to look up LoadVars() to do that.
Nothing too difficult but like I said you’ll have a harder time getting the php part working.