The basic query format could be something like:
UPDATE table_name SET balance = balance - :withdrawl_amount WHERE user_id = :user_id
But then you would want additional logic/queries to make sure that balance won't drop below zero (presumably?).
But somewhat to what ginerjm was referring to: If you are going to actually be taking/sending real money from users, you better have some real strong knowledge about what you are doing, including all the security issues and legal requirements -- not just throwing together a bunch of PHP/MySQL code that you hope will work. Payment systems on sites like Paypal and Amazon were not thrown together in a week by some guy sitting up late at his computer: they were built by teams of developers, testers, subject matter experts, etc.; because if you do that sort of thing wrong, you can be totally screwed.