如何使用PHP中的多个工作表生成Excel文档？

require_once 'PHPExcel/PHPExcel.php';

$objPHPExcel = new PHPExcel();
$objPHPExcel->getProperties()->setCreator("Your Name")
                             ->setLastModifiedBy("Your Name")
                             ->setTitle("Excel Document")
                             ->setSubject("Excel Document")
                             ->setDescription("Generated by PHPExcel")
                             ->setKeywords("excel php")
                             ->setCategory("Test");

$objPHPExcel->setActiveSheetIndex(0);
$objPHPExcel->getActiveSheet()->setTitle('Sheet1');

$objPHPExcel->getActiveSheet()->setCellValue('A1', 'Hello');
$objPHPExcel->getActiveSheet()->setCellValue('B1', 'World');

$objPHPExcel->createSheet();
$objPHPExcel->setActiveSheetIndex(1);
$objPHPExcel->getActiveSheet()->setTitle('Sheet2');

$objPHPExcel->getActiveSheet()->setCellValue('A1', 'Foo');
$objPHPExcel->getActiveSheet()->setCellValue('B1', 'Bar');

$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel2007');
$objWriter->save('example.xlsx');