PHP money_format
From the PHP.net comment:
If money_format doesn't seem to be working properly, make sure you are defining a valid locale. For example, on Debian, 'en_US' is not a valid locale - you need 'en_US.UTF-8' or 'en_US.ISO-8559-1'.
This was frustrating me for a while. Debian has a list of valid locales at /usr/share/i18n/SUPPORTED; find yours there if it's not working properly.