These functions allow you to work with arbitrary-length integers using GNU MP library. In order to have these functions available, you must compile PHP with GMP support by using the --with-gmp option.
You can download the GMP library from http://www.swox.com/gmp/. This site also has the GMP manual available.
You will need GMP version 2 or better to use these functions. Some functions may require more recent version of the GMP library.
These functions have been added in PHP 4.0.4.
Nota: Most GMP functions accept GMP number arguments, defined as resource below. However, most of these functions will accept also numeric and string arguments, given it's possible to convert the latter to number. Also, if there's faster function that can operate on integer arguments, it would be used instead of slower function when supplied arguments are integers. This is done transparently, so the bootom line is that you can use integers in every function that expects GMP number. See also gmp_init() function.
This will calculate factiorial of 1000 (pretty big number) very fast.