如何显示Mysql php结果json格式？

$servername = "数据库服务器地址";
$username = "数据库用户名";
$password = "数据库密码";
$dbname = "数据库名称";

// 使用mysqli连接数据库
$conn = new mysqli($servername, $username, $password, $dbname);

// 使用PDO连接数据库
$dsn = "mysql:host=$servername;dbname=$dbname;charset=utf8mb4";
$conn = new PDO($dsn, $username, $password);

$sql = "SELECT * FROM 表名";
$result = $conn->query($sql);

$rows = array();
while ($row = $result->fetch_assoc()) {
    $rows[] = $row;
}

$json = json_encode($rows);

header('Content-Type: application/json');
echo $json;