Number_format() is a remarkably helpful function that takes a minimum of one parameter, the number to format, and returns that same number with grouped thousands. There are two function prototypes for number_format() as you either pass it one, two, or four parameters - passing it one or two fits the first prototype, and passing four fits the second.
So, if you pass number_format() a parameter of "1234567", it will return "1,234,567". By default, number_format() rounds fractions - 1234567.89 becomes 1,234,568. However, you can change this by specifying the second parameter, which is the number of decimal places to include. Parameter three allows you to choose the character to use as your decimal point, and parameter four allows you to choose the character to use as your thousands separator. Here is how it all looks in PHP:
After running that script, $a will be set to 12,346, $b will be set to 12,345.679, and $c will be set to 12.345,6789 (periods used to separate thousands, and commas used for the decimal point, east European-style).
As you can imagine, number_format() is incredibly useful when it comes to formatting money for checkout pages in shopping baskets, although it is useful anywhere you need to represent large numbers - adding a thousand separator invariably makes things easier to read.