Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/DateTime.php
public static function MONTHOFYEAR($dateValue = 1)
    {
        $dateValue = Functions::flattenSingleValue($dateValue);

        if (empty($dateValue)) {
            $dateValue = 1;
        }
        if (is_string($dateValue = self::getDateValue($dateValue))) {
            return Functions::VALUE();
        } elseif ($dateValue < 0.0) {
            return Functions::NAN();
        }

        // Execute function
        $PHPDateObject = Date::excelToDateTimeObject($dateValue);

        return (int) $PHPDateObject->format('n');
    }
public static function MONTHOFYEAR($dateValue = 1)
    {
        $dateValue = Functions::flattenSingleValue($dateValue);

        if (empty($dateValue)) {
            $dateValue = 1;
        }
        if (is_string($dateValue = self::getDateValue($dateValue))) {
            return Functions::VALUE();
        } elseif ($dateValue < 0.0) {
            return Functions::NAN();
        }

        // Execute function
        $PHPDateObject = Date::excelToDateTimeObject($dateValue);

        return (int) $PHPDateObject->format('n');
    }
MONTHOFYEAR.

Returns the month of a date represented by a serial number.
The month is given as an integer, ranging from 1 (January) to 12 (December).

Excel Function:
MONTH(dateValue)

@param mixed $dateValue Excel date serial value (float), PHP date timestamp (integer),
PHP DateTime object, or a standard date string

@return int Month of the year