Display a number as currency with the function format_currency(amount, currency_code, display_mode).
See this table on Wikipedia for currency_code options.
There are three options for display_mode:
0 (default): Show the amount with the appropriate currency symbol.
$123.75
1: Show the symbol and distinguish from other currencies with the same symbol.
US$123.75 (which distinguishes US Dollars from Canadian Dollars, for example)
2: Show the amount with the country code and symbol.
USD $123.75