Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xls/Worksheet.php
private function writeNumber($row, $col, $num, $xfIndex)
    {
        $record = 0x0203; // Record identifier
        $length = 0x000E; // Number of bytes to follow

        $header = pack('vv', $record, $length);
        $data = pack('vvv', $row, $col, $xfIndex);
        $xl_double = pack('d', $num);
        if (self::getByteOrder()) { // if it's Big Endian
            $xl_double = strrev($xl_double);
        }

        $this->append($header . $data . $xl_double);

        return 0;
    }
private function writeNumber($row, $col, $num, $xfIndex)
    {
        $record = 0x0203; // Record identifier
        $length = 0x000E; // Number of bytes to follow

        $header = pack('vv', $record, $length);
        $data = pack('vvv', $row, $col, $xfIndex);
        $xl_double = pack('d', $num);
        if (self::getByteOrder()) { // if it's Big Endian
            $xl_double = strrev($xl_double);
        }

        $this->append($header . $data . $xl_double);

        return 0;
    }
Write a double to the specified row and column (zero indexed).
An integer can be written as a double. Excel will display an
integer. $format is optional.

Returns  0 : normal termination
-2 : row or column out of range

@param int $row Zero indexed row
@param int $col Zero indexed column
@param float $num The number to write
@param mixed $xfIndex The optional XF format

@return int