that's basicly like saying "I want to create a dynamic site, how does that work?" ... in your case, you need to setup a database, create the corresponding fields, create the PHP code to read and then to update the fields - don't forget to check the vars meanwhile btw.
I'd suggest reading a PHP/mySQL book or a tutorial on the web on how to get started first - as we can't really write the whole thing for you.
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