As most people know, it's a horrible idea to use floating point numbers to represent currencies. For a more detailed explanation, see: 
http://stackoverflow.com/questions/3730019/why-not-use-double-or-float-to-represent-currency

Let's get rid of rounding issues forever, and implement BC Math (Arbitrary Precision Mathemathics) to do currency related calculations:
http://www.php.net/manual/en/book.bc.php