Wie zentriere ich den Text "test"?

Das ist mein Code:

<?php
    /** Error reporting */
    error_reporting(E_ALL);
    ini_set('display_errors', TRUE);
    ini_set('display_startup_errors', TRUE);
    date_default_timezone_set('Europe/London');

    /** Include PHPExcel */
    require_once '../Classes/PHPExcel.php';

    $objPHPExcel = new PHPExcel();
    $sheet = $objPHPExcel->getActiveSheet();
    $sheet->setCellValueByColumnAndRow(0, 1, "test");
    $sheet->mergeCells('A1:B1');
    $objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
    $objWriter->save("test.xlsx");

Excel-Dokument ausgeben: