XXV. GMP functions
These functions allow you to work with arbitrarylength integers using the GNU MP library. In order to have these functions available, you must compile PHP with GMP support by using the withgmp 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.
Note: Most GMP functions accept GMP number arguments, defined as resource below. However, most of these functions will also accept numeric and string arguments, given that it is possible to convert the latter to a number. Also, if there is a faster function that can operate on integer arguments, it would be used instead of the slower function when the supplied arguments are integers. This is done transparently, so the bottom line is that you can use integers in every function that expects GMP number. See also the gmp_init() function.
Example 1. Factorial function using GMP
<?php function fact ($x) { if ($x <= 1) return 1; else return gmp_mul ($x, fact ($x1)); } print gmp_strval (fact (1000)) . "\n"; ?> 

This will calculate factiorial of 1000 (pretty big number) very fast.