PHP JSON不显示MySQL结果

<?php
// 连接到MySQL数据库
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database";
$conn = new mysqli($servername, $username, $password, $dbname);

// 检查连接是否成功
if ($conn->connect_error) {
    die("连接失败: " . $conn->connect_error);
}

// 执行查询语句
$sql = "SELECT * FROM table";
$result = $conn->query($sql);

// 检查查询结果是否为空
if ($result->num_rows > 0) {
    // 将查询结果转换为关联数组
    $rows = array();
    while ($row = $result->fetch_assoc()) {
        $rows[] = $row;
    }

    // 将查询结果转换为JSON字符串
    $json = json_encode($rows);

    // 设置HTTP头信息为JSON格式
    header('Content-Type: application/json');

    // 输出JSON字符串
    echo $json;
} else {
    echo "查询结果为空";
}

// 关闭数据库连接
$conn->close();
?>