ISFORMULA() not recognized / throws exception

[billynoah]

ISFORMULA() not recognized and throws exception. Steps to reproduce. Create a new spreadsheet. In the cell A1 enter

=ISFORMULA(B1)

The result should be FALSE and is valid. Try loading the spreadsheet in PhpSpreadsheet and read cells like this:

$spreadsheet = \PhpOffice\PhpSpreadsheet\IOFactory::load($filename);
$rows = $spreadsheet->getSheet($sheet)->toArray();

Results in an exception:

Sheet1!A1 -> undefined named range / function '_xlfn.ISFORMULA'

Related: PHPOffice/PHPExcel#1136

formula_test.xlsx

[PowerKiKi]

I can reproduce it with:

<?php

require __DIR__ . '/vendor/autoload.php';

use PhpOffice\PhpSpreadsheet\Spreadsheet;

$spreadsheet = new Spreadsheet();
$sheet = $spreadsheet->getActiveSheet();
$cell = $sheet->getCell('A1');
$cell->setValue('=ISFORMULA(B1)');
var_dump($cell->getCalculatedValue());

Would you like to submit a PR to add support for ISFORMULA ?