请写出完整的代码示例给我

好的，下面是一个完整的使用 PHPExcel 导出数据库大量数据的示例代码：

<?php
require 'PHPExcel/PHPExcel.php';

// 连接到数据库
$host = 'localhost';
$dbname = 'test';
$username = 'root';
$password = '';
$dsn = "mysql:host=$host;dbname=$dbname;charset=utf8";
$options = array(
    PDO::ATTR_ERRMODE => PDO::ERRMODE_EXCEPTION,
    PDO::ATTR_DEFAULT_FETCH_MODE => PDO::FETCH_ASSOC,
);
$db = new PDO($dsn, $username, $password, $options);

// 查询需要导出的数据
$sql = "SELECT * FROM users";
$stmt = $db->prepare($sql);
$stmt->execute();
$result = $stmt->fetchAll();

// 创建 PHPExcel 对象
$objPHPExcel = new PHPExcel();

// 设置文件属性
$objPHPExcel->getProperties()->setCreator("Your Name")
                             ->setLastModifiedBy("Your Name")
                             ->setTitle("Export Data")
                             ->setSubject("Export Data")
                             ->setDescription("Export Data")
                             ->setKeywords("Excel")
                             ->setCategory("Export Data");

// 设置表头
$fields = $stmt->fetchAll(PDO::FETCH_ASSOC);
$column = 0;
foreach ($fields as $field) {
    $objPHPExcel->setActiveSheetIndex(0)
                ->setCellValueByColumnAndRow($column, 1, $field['COLUMN_NAME']);
    $column++;
}

// 将数据添加到 Excel 文件中
$row = 2;
while ($data = $result->fetch(PDO::FETCH_ASSOC)) {
    $column = 0;
    foreach ($data as $value) {
        $objPHPExcel->setActiveSheetIndex(0)
                    ->setCellValueByColumnAndRow($column, $row, $value);
        $column++;
    }
    $row++;
}

// 输出 Excel 文件
header('Content-Type: application/vnd.ms-excel');
header('Content-Disposition: attachment;filename="export.xls"');
header('Cache-Control: max-age=0');
$objWriter = PHPExcel_IOFactory::createWriter($objPHPExcel, 'Excel5');
$objWriter->save('php://output');