A simple poisson-distribution function, which only works for relatively small numbers (maximum of ~100/100 for most systems):

<?php

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

function poisson($chance, $occurrence)
{
        $e = exp(1);

        $a = pow($e, (-1 * $chance));
        $b = pow($chance,$occurrence);
        $c = factorial($occurrence);

        return $a * $b / $c;
}

$test = poisson(100,100);
echo $test . "\n";

?>