what function to update the formulas of the excel file? - spreadsheet

Question

with what function to update the formulas of the excel file?
if I modify the value of cell C2 and C3, the formula of cell C4 which is equal to sum C2: C3 does not update
what is missing in my code so that the calculation is regenerated?
in my source file I have a lot of complex formulas that I would like to be updated automatically after changing the value of some cells
thank you
?php
session_start();
header('Content-Type: text/html; charset=UTF-8'); /*obligatoire sur chaque page*/
//call the autoload
require 'PhpSpreadSheet/vendor/autoload.php';
//include the classes needed to create and write .xlsx file
use PhpOffice\PhpSpreadsheet\Spreadsheet;
use PhpOffice\PhpSpreadsheet\Reader\IReader;
use PhpOffice\PhpSpreadsheet\IOFactory;
use PhpOffice\PhpSpreadsheet\Writer\Xlsx;
//create directly an object instance of the IOFactory class, and load the xlsx file
$fxls ='excel-file_1.xlsx';
$spreadsheet = \PhpOffice\PhpSpreadsheet\IOFactory::load($fxls);
//read excel data and store it into an array
$xls_data1 = $spreadsheet->getActiveSheet()->getCell('C2');
$xls_data2 = $spreadsheet->getActiveSheet()->getCell('C3');
$xls_data3 = $spreadsheet->getActiveSheet()->getCell('C4')->getCalculatedValue();
echo $xls_data1.' | '.$xls_data2.' | '.$xls_data3;
//result: 4000 | 15000 | 19000 => it's ok! C4=C2+C3
echo '<br/>';
$spreadsheet->getActiveSheet()->setCellValue('C2', '5000');
$spreadsheet->getActiveSheet()->setCellValue('C3', '17896');
$xls_data4 = $spreadsheet->getActiveSheet()->getCell('C2');
$xls_data5 = $spreadsheet->getActiveSheet()->getCell('C3');
$xls_data6 = $spreadsheet->getActiveSheet()->getCell('C4')->getCalculatedValue();
echo $xls_data4.' | '.$xls_data5.' | '.$xls_data6;
//result: 5000 | 17896 | 19000 => it's not good => not 19000 but 22896 (C4=C2+C3)
?>

Answer 1

I found a tip to solve my problem but another problem arises
the saved file cannot be modified (read only).
do you know a solution so that the saved file can remain editable?
please
thank you
<?php
session_start();
header('Content-Type: text/html; charset=UTF-8');
require 'PhpSpreadSheet/vendor/autoload.php';
use PhpOffice\PhpSpreadsheet\Spreadsheet;
use PhpOffice\PhpSpreadsheet\Reader\IReader;
use PhpOffice\PhpSpreadsheet\IOFactory;
use PhpOffice\PhpSpreadsheet\Writer\Xlsx;
$fxls ='file.xls';
$spreadsheet = PhpOffice\PhpSpreadsheet\IOFactory::load($fxls);
$spreadsheet->getActiveSheet()->setCellValue('H4', '5'); $spreadsheet->getActiveSheet()->setCellValue('I4', '0');
$spreadsheet->getActiveSheet()->setCellValue('H5', '2'); $spreadsheet->getActiveSheet()->setCellValue('I5', '1');
$spreadsheet->getActiveSheet()->setCellValue('H6', '3'); $spreadsheet->getActiveSheet()->setCellValue('I6', '3');
$writer = PhpOffice\PhpSpreadsheet\IOFactory::createWriter($spreadsheet, "Xlsx");
$writer->save($fxls); (file read only?)
$spreadsheet = PhpOffice\PhpSpreadsheet\IOFactory::load($fxls);
//$xls_data4 = $spreadsheet->getActiveSheet()->getCell('C8');
//$xls_data5 = $spreadsheet->getActiveSheet()->getCell('Z8');
$xls_data1 = $spreadsheet->getActiveSheet()->getCell('V8')->getCalculatedValue();
$xls_data2 = $spreadsheet->getActiveSheet()->getCell('Z8')->getCalculatedValue();
echo $xls_data1.'<br/>';
echo $xls_data2.'<br/>';
?>