PHPOffice / PhpSpreadsheet

        $ext_ref = preg_replace("/'$/", '', $ext_ref); // Remove trailing ' if any.

        // Check if there is a sheet range eg., Sheet1:Sheet2.

        if (preg_match('/:/', $ext_ref)) {

            list($sheet_name1, $sheet_name2) = explode(':', $ext_ref);

            $sheet1 = $this->getSheetIndex($sheet_name1);

            if ($sheet1 == -1) {

                throw new \PhpOffice\PhpSpreadsheet\Writer\Exception("Unknown sheet name $sheet_name1 in formula");

            }

            $sheet2 = $this->getSheetIndex($sheet_name2);

            if ($sheet2 == -1) {

                throw new \PhpOffice\PhpSpreadsheet\Writer\Exception("Unknown sheet name $sheet_name2 in formula");

            }

            // Reverse max and min sheet numbers if necessary

            if ($sheet1 > $sheet2) {

                list($sheet1, $sheet2) = [$sheet2, $sheet1];

            }

        } else { // Single sheet name only.

            $sheet1 = $this->getSheetIndex($ext_ref);

            if ($sheet1 == -1) {

                throw new \PhpOffice\PhpSpreadsheet\Writer\Exception("Unknown sheet name $ext_ref in formula");

            }

            $sheet2 = $sheet1;

        }

        // References are stored relative to 0xFFFF.

        $offset = -1 - $sheet1;

        $ext_ref = str_replace('\'\'', '\'', $ext_ref); // Replace escaped '' with '

        // Check if there is a sheet range eg., Sheet1:Sheet2.

        if (preg_match('/:/', $ext_ref)) {

            list($sheet_name1, $sheet_name2) = explode(':', $ext_ref);

            $sheet1 = $this->getSheetIndex($sheet_name1);

            if ($sheet1 == -1) {

                throw new \PhpOffice\PhpSpreadsheet\Writer\Exception("Unknown sheet name $sheet_name1 in formula");

            }

            $sheet2 = $this->getSheetIndex($sheet_name2);

            if ($sheet2 == -1) {

                throw new \PhpOffice\PhpSpreadsheet\Writer\Exception("Unknown sheet name $sheet_name2 in formula");

            }

            // Reverse max and min sheet numbers if necessary

            if ($sheet1 > $sheet2) {

                list($sheet1, $sheet2) = [$sheet2, $sheet1];

            }

        } else { // Single sheet name only.

            $sheet1 = $this->getSheetIndex($ext_ref);

            if ($sheet1 == -1) {

                throw new \PhpOffice\PhpSpreadsheet\Writer\Exception("Unknown sheet name $ext_ref in formula");

            }

            $sheet2 = $sheet1;

        }

        // assume all references belong to this document

        $supbook_index = 0x00;