Binomial PHP

<?php

// if GMP extension is not enabled
// we must create a gmp_fact function
if(!function_exists('gmp_fact')) {
    function gmp_fact($x) {
        $result = 1;
        for($i = $x; $i > 1; $i--) {
            $result *= $x;
            $x--;
        }
        return $result;
    }
}

$varn = 5;
$varx = 1;
$varp = 0.15;
$varq = 0.85;

// calculate n combination of x
$comb = gmp_fact($varn)/(gmp_fact($varx)*gmp_fact($varn-$varx));

// calculate probability
$prob = $comb*pow($varp, $varx)*pow($varq, $varn-$varx);

// result
echo $prob;