In mathematics, the factorial of a non-negative integer n, denoted by n!, is the product of all positive integers less than or equal to n. There are 2 ways to archive it, by iterative function or by recursive function.

factorial_iterative_recursive.php

<?php
function factorial_iterative($n)
{
    $k = 1;
    for($i = $n; $i > 1; $i--)
    {
        $k *= $i;
    }

    return $k;
}
function factorial_recursive($n)
{
    if($n < 2)
    {
        return 1;
    }
    else
    {
        return ($n * factorial_recursive($n - 1));
    }
}
echo factorial_iterative(5)."\n";
echo factorial_iterative(10)."\n";
echo factorial_iterative(15)."\n";
echo "\n";
echo factorial_recursive(5)."\n";
echo factorial_recursive(10)."\n";
echo factorial_recursive(15)."\n";