Returns the inverse standard normal cumulative distribution ( 0<y<1 ) (PHP)

normDistInv.php

<?
    /**
     * @param mixed $y
     * @return string
     */
    public function normDistInv($y)
    {
        // Load tabulated values in an array
        $values = config('table');
        // Discriminate upon whether $y is between 0 and 1, then upon its position relative to 0.5
        $y = number_format($y, 2);
        if ($y <= 0 || $y >= 1) {
            $output = false;
        } elseif ($y <= 0.5) {
            // find the largest index which value is smaller than $y:
            // filter array for values higher than $y
            $smaller = [];
            while (list($key, $value) = $this->myEach($values)) {
                if ($value <= $y) {
                    $smaller[$key] = $value;
                }
            }
            // order $values in decreasing terms of the $values
            krsort($smaller, SORT_NUMERIC);
            reset($smaller);
            $x1 = (string) key($smaller);
            $x2 = (string) ($x1 + 0.01);
            // interpolate
            $output = $x1 + ($y - $values[$x1]) / ($values[$x2] - $values[$x1]) * 0.01;
        } else {
            // meaning $x between 0.5 and 1
            $output = -$this->normDistInv(1 - $y);
        }

        return number_format($output, 4);
    }