php mysql输出json格式

<?php
// 数据库连接参数
$servername = "localhost";
$username = "username";
$password = "password";
$dbname = "database_name";

// 创建连接
$conn = new mysqli($servername, $username, $password, $dbname);

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

// SQL查询
$sql = "SELECT id, name FROM users";
$result = $conn->query($sql);

// 检查查询结果
if ($result->num_rows > 0) {
    // 输出每行数据
    $rows = array();
    while($row = $result->fetch_assoc()) {
        $rows[] = $row;
    }
    // 将数组转换为JSON格式
    echo json_encode($rows);
} else {
    echo "0 结果";
}
$conn->close();
?>

$conn->set_charset("utf8mb4");

if ($result->num_rows > 0) {
    $rows = array();
    while($row = $result->fetch_assoc()) {
        // 处理NULL值
        foreach ($row as $key => $value) {
            if (is_null($value)) {
                $row[$key] = ""; // 或者其他默认值
            }
        }
        $rows[] = $row;
    }
    echo json_encode($rows);
} else {
    echo json_encode(array()); // 输出空数组而不是字符串"0 结果"
}