function factorial($x)
{
    if (0 == $x) {
        return 1;
    }
    return $x * factorial($x - 1);
}

function factorial($i)
{
    if ($i <= 1) {
        return 1;
    }
    return $i * factorial($i - 1);
}

function factorial($n)
{
    if ($n < 2) {
        return $n;
    }
    return $n * factorial($n - 1);
}

function getAllPermutations($numberOfChosenElements, $setSize)
{
    //r = number of Chosen Elements
    //n = set Size
    //number of Combinations = n!/((n-r)!(r!))
    //returns an array of combinations
    $n = intval($setSize);
    $r = intval($numberOfChosenElements);
    //calculate total combinations (inc. non unique)
    $totalCombinations = factorial($setSize);
    //unique combinations
    $uniqueCombinations = factorial($n) / (factorial($n - $r) * factorial($r));
    //permutations = (number of Chosen Elements)^ set Size
    $permutations = pow($n, $r);
    $array = array();
    //$uniqueList = array();
    //echo  "Total Combinations: " . $totalCombinations . "<br/>";
    echo "Permutations: " . $permutations . "<br/>";
    $i = 0;
    for ($i = 0; $i < $r; $i++) {
        //initialize array to all 0's
        $array[$i] = 0;
    }
    $i = 0;
    //start combo checking!
    Permutation(0, $array, $n);
}

function factorial($x)
{
    if ($x <= 1) {
        return TRUE;
    }
    return $x * factorial($x - 1);
}

function factorial($n)
{
    if ($n == 1) {
        return 1;
    }
    return $n * factorial($n - 1);
}

/**
 * factorial.
 * 
 * @param  int $n The number to calculate factorial for
 * 
 * @return int    The factorial of $n
 * 
 * @throws InvalidArgumentException  If input is not valid number
 */
function factorial($n)
{
    if (!is_numeric($n)) {
        throw new \InvalidArgumentException(sprintf('%s is not valid number', $n));
    }
    return $n < 2 ? 1 : $n * factorial($n - 1);
}

function factorial($number)
{
    if ($number === 1) {
        return $number;
    }
    return factorial($number - 1) * $number;
}

function factorial($n)
{
    if ($n == 0) {
        return 1;
    }
    return factorial($n - 1) * $n;
}

function createResultArray($ro, $steps)
{
    $resultArray = array();
    for ($k = 0; $k < $steps; $k++) {
        $resultArray[] = pow($ro, $k) * exp(-$ro) / factorial($k);
    }
    return $resultArray;
}

function calc($n)
{
    $res = 0;
    for ($i = 0; $i <= $n; $i++) {
        $res += 1 / factorial($i);
    }
    return $res;
}

function factorial($num)
{
    if ($num == 1) {
        return 1;
    }
    echo $num . "<br />";
    return $num * factorial($num - 1);
}

function factorial($n)
{
    if ($n <= 1) {
        return 1;
    } else {
        return $n * factorial($n - 1);
    }
}

function factorial($x)
{
    if ($x <= 1) {
        return 1;
    } else {
        return $x * factorial($x - 1);
    }
}

/**
 * @param int $number
 */
function factorial($number)
{
    if ($number < 2) {
        return 1;
    } else {
        return factorial($number - 1) * $number;
    }
}

function factorial($x)
{
    static $cache = [];
    if (isset($cache[$x])) {
        return $cache[$x];
    }
    $cache[$x] = $x == 1 ? 1 : $x * factorial($x - 1);
    return $cache[$x];
}

function factorial($x)
{
    assert(gettype($x) == "integer");
    if ($x == 1) {
        return 1;
    } else {
        return $x * factorial($x - 1);
    }
}

/**
 * @param $number
 *
 * @return int
 */
function factorial($number)
{
    if ($number < 2) {
        return 1;
    } else {
        echo $number . '*' . $number - 1;
        return $number * factorial($number - 1);
    }
}

function factorial($numb)
{
    if ($numb > 1) {
        return $numb * factorial($numb - 1);
    }
    if ($numb < 0) {
        return -1;
    }
    return 1;
}

/**
 * <em>Calculating factorial for a given number</em>
 *
 * @param  int $num an integer of a value to be calculated
 * @return int $result  a final factorial result
 */
function factorial($num)
{
    if ($num == 0) {
        return 1;
    } else {
        $result = $num * factorial($num - 1);
        //Using recursive function.
    }
    return $result;
}