number_format() returns a formatted version of number. This function accepts either one, two or four parameters (not three):
If only one parameter is given, Number will be formatted without decimals, but with a comma (",") between every group of thousands.
If two parameters are given, number will be formatted with decimals decimals with a dot (".") in front, and a comma (",") between every group of thousands.
If all four parameters are given, number will be formatted with decimals decimals, dec_point instead of a dot (".") before the decimals and thousands_sep instead of a comma (",") between every group of thousands.
Note: Only the first character of thousands_sep is used. For example, if you use foo as thousands_sep on the number 1000, number_format() will return 1f000.