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

        if (($significance === null) &&
            (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_GNUMERIC)) {
            $significance = $number / abs($number);
        }

        if ((is_numeric($number)) && (is_numeric($significance))) {
            if ($significance == 0.0) {
                return Functions::DIV0();
            } elseif ($number == 0.0) {
                return 0.0;
            } elseif (self::SIGN($number) == self::SIGN($significance)) {
                return floor($number / $significance) * $significance;
            }

            return Functions::NAN();
        }

        return Functions::VALUE();
    }
public static function FLOOR($number, $significance = null)
    {
        $number = Functions::flattenSingleValue($number);
        $significance = Functions::flattenSingleValue($significance);

        if (($significance === null) &&
            (Functions::getCompatibilityMode() == Functions::COMPATIBILITY_GNUMERIC)) {
            $significance = $number / abs($number);
        }

        if ((is_numeric($number)) && (is_numeric($significance))) {
            if ($significance == 0.0) {
                return Functions::DIV0();
            } elseif ($number == 0.0) {
                return 0.0;
            } elseif (self::SIGN($number) == self::SIGN($significance)) {
                return floor($number / $significance) * $significance;
            }

            return Functions::NAN();
        }

        return Functions::VALUE();
    }
FLOOR.

Rounds number down, toward zero, to the nearest multiple of significance.

Excel Function:
FLOOR(number[,significance])

@category Mathematical and Trigonometric Functions

@param float $number Number to round
@param float $significance Significance

@return float Rounded Number