Why a Money library for PHP?

Also see http://blog.verraes.net/2011/04/fowler-money-pattern-in-php/

This is a PHP implementation of the Money pattern, as described in [Fowler2002] :

A large proportion of the computers in this world manipulate money, so it’s always puzzled me that money isn’t actually a first class data type in any mainstream programming language. The lack of a type causes problems, the most obvious surrounding currencies. If all your calculations are done in a single currency, this isn’t a huge problem, but once you involve multiple currencies you want to avoid adding your dollars to your yen without taking the currency differences into account. The more subtle problem is with rounding. Monetary calculations are often rounded to the smallest currency unit. When you do this it’s easy to lose pennies (or your local equivalent) because of rounding errors.
[Fowler2002]Fowler, M., D. Rice, M. Foemmel, E. Hieatt, R. Mee, and R. Stafford, Patterns of Enterprise Application Architecture, Addison-Wesley, 2002. http://martinfowler.com/books.html#eaa

The goal

Implement a reusable Money class in PHP, using all the best practices and taking care of all the subtle intricacies of handling money. I hope to add a lot more features, such as dealing with major units and subunits in currencies, currency conversion, string formatting and parsing, ... Other ideas include integration with Doctrine2, which should make it easier to store money in a database transparently.