Return Probability From the Table (PHP)

prob.php

<? 

    public function returnProbabilityFromTheTable($zValue)
    {
        return $this->CDF($zValue);
    }

    public function sgn($x)
    {
        if ($x < 0) {
            return -1;
        }

        return 1;
    }

    public function erf($x)
    {
        $e = exp(-$x * $x);
        $e2 = exp(-$x * $x * 2);

        $q = sqrt(pi()) / 2 + 31 * $e / 200 - 341 * $e2 / 8000;

        return 2 * $this->sgn($x) * sqrt(1 - $e) * $q / sqrt(pi());
    }

    public function CDF($x)
    {
        return (1 + $this->erf($x / sqrt(2))) / 2;
    }