Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/Statistical.php
public static function POISSON($value, $mean, $cumulative)
    {
        $value = Functions::flattenSingleValue($value);
        $mean = Functions::flattenSingleValue($mean);

        if ((is_numeric($value)) && (is_numeric($mean))) {
            if (($value < 0) || ($mean <= 0)) {
                return Functions::NAN();
            }
            if ((is_numeric($cumulative)) || (is_bool($cumulative))) {
                if ($cumulative) {
                    $summer = 0;
                    $floor = floor($value);
                    for ($i = 0; $i <= $floor; ++$i) {
                        $summer += pow($mean, $i) / MathTrig::FACT($i);
                    }

                    return exp(0 - $mean) * $summer;
                }

                return (exp(0 - $mean) * pow($mean, $value)) / MathTrig::FACT($value);
            }
        }

        return Functions::VALUE();
    }
public static function POISSON($value, $mean, $cumulative)
    {
        $value = Functions::flattenSingleValue($value);
        $mean = Functions::flattenSingleValue($mean);

        if ((is_numeric($value)) && (is_numeric($mean))) {
            if (($value < 0) || ($mean <= 0)) {
                return Functions::NAN();
            }
            if ((is_numeric($cumulative)) || (is_bool($cumulative))) {
                if ($cumulative) {
                    $summer = 0;
                    $floor = floor($value);
                    for ($i = 0; $i <= $floor; ++$i) {
                        $summer += pow($mean, $i) / MathTrig::FACT($i);
                    }

                    return exp(0 - $mean) * $summer;
                }

                return (exp(0 - $mean) * pow($mean, $value)) / MathTrig::FACT($value);
            }
        }

        return Functions::VALUE();
    }
POISSON.

Returns the Poisson distribution. A common application of the Poisson distribution
is predicting the number of events over a specific time, such as the number of
cars arriving at a toll plaza in 1 minute.

@param float $value
@param float $mean Mean Value
@param bool $cumulative

@return float