An efficient PHP script using GMP to calculate factorials.

fac.php

<?php
if(!isset($argv[1]))
{
	die("Syntax: php fac.php <num>\n");
}
$start_time = microtime(true);
$fac = $one = gmp_init("1");
$i = gmp_init("1");
$limit = gmp_init($argv[1]);
do
{
	$i = gmp_add($i, $one);
	$fac = gmp_mul($fac, $i);
}
while(gmp_cmp($i, $limit) != 0);
echo gmp_strval($limit)."! = ".gmp_strval($fac)."\n";
echo "Calculated in ".(microtime(true) - $start_time)." seconds.\n";

amazing