How to set active sheet without loading an xlsx file?

You do, of course, need to create/add additional worksheets to be able to change the active sheet: using new PHPExcel() will only create a workbook containing a single sheet.

You can set the active sheet using either the sheet index (sheets are indexed from 0);

$objPHPExcel->setActiveSheetIndex(2);

or by name

$objPHPExcel->setActiveSheetIndexByName('My Second Sheet');

Adding a new sheet using either the createSheet() or addSheet() methods will automatically set that new worksheet to the active worksheet. By default, any new worksheet will be given a name comprising the word "Worksheet" and a number until you use setTitle() to change it.

Add below function into Excel.php class file:

function setActiveSheet($sheetnumber) {
        $this->objPHPExcel->setActiveSheetIndex($sheetnumber);
    }

then call that function like this :

$phpExcel->setActiveSheet(0);