Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/MathTrig.php
public static function ROUNDDOWN($number, $digits)
    {
        $number = Functions::flattenSingleValue($number);
        $digits = Functions::flattenSingleValue($digits);

        if ((is_numeric($number)) && (is_numeric($digits))) {
            $significance = pow(10, (int) $digits);
            if ($number < 0.0) {
                return ceil($number * $significance) / $significance;
            }

            return floor($number * $significance) / $significance;
        }

        return Functions::VALUE();
    }
public static function ROUNDDOWN($number, $digits)
    {
        $number = Functions::flattenSingleValue($number);
        $digits = Functions::flattenSingleValue($digits);

        if ((is_numeric($number)) && (is_numeric($digits))) {
            $significance = pow(10, (int) $digits);
            if ($number < 0.0) {
                return ceil($number * $significance) / $significance;
            }

            return floor($number * $significance) / $significance;
        }

        return Functions::VALUE();
    }
ROUNDDOWN.

Rounds a number down to a specified number of decimal places

@param float $number Number to round
@param int $digits Number of digits to which you want to round $number

@return float Rounded Number