money is a PHP library that implements Martin Fowler’s design pattern for money. See the official documentation and this blog post for more information.
Some of the issues addresses by this library are the binding of the money amount to the currency, and currency conversion operations. If your code needs to deal with multiple currencies, you’ll definitely benefit from this library. But even if you are dealing with a single currency, it’s still pretty handy, making the code more transparent and future proof.