External workbook name not in formula

[Benny44]

I understand (from digging through the codebase) that a formula with a reference to a cell in an external workbook does not work. What I don't get, however, is why the name of the workbook is filtered out and replaced by a number. F.e. in the minimal working example below, I have set cell A1 in basic2 to refer to A1 in basic. This reads in excel as the following formula: =[basic.xlsx]Sheet1!$A$1. In PHP, however, I obtain =[1]Sheet1!$A$1 using the code below. Can someone tell me where this conversion is made and/or how I could retrieve the filename of the external workbook? This would help me, even though the calculation can not be run.

Interestingly, I noted that if I include a reference to another (different) workbook in basic2, this will be read as [2], so that is why I suspect a conversion between external workbook names and numbers is made at some point.

require '../vendor/autoload.php';
use PhpOffice\PhpSpreadsheet\Spreadsheet;

$reader = new \PhpOffice\PhpSpreadsheet\Reader\Xlsx();
$reader->setIgnoreRowsWithNoCells(true);
$reader->setReadDataOnly(true);

$spreadsheet = $reader->load("./basic2.xlsx");

$cell = $spreadsheet->getActiveSheet()->getCell("A1");

$cell->getStyle()->setQuotePrefix(false);
echo $cell->getValue() . "\n";

basic.xlsx
basic2.xlsx

[Benny44]

Nevermind, I just found instructions to do exactly this involving changing the extension to zip and then unzipping the file in a reply to another issue, #1607.