怎么让服务器访问数据库

 // 创建连接

 $conn = new mysqli($servername, $username, $password, $dbname);

 // 检查连接是否成功

 if ($conn->connect_error) {

     die("连接失败: " . $conn->connect_error);

 }

 echo "连接成功";

 ?>

 ```

 # 创建连接

 client = MongoClient('mongodb://localhost:27017/')

 # 获取数据库

 db = client['database']

 # 获取集合

 collection = db['collection']

 # 执行查询等操作

 # ...

 ```

 public class Main {

     public static void main(String[] args) {

         String url = "jdbc:postgresql://localhost/database";

         String user = "username";

         String password = "password";

         try {

             // 创建连接

             Connection conn = DriverManager.getConnection(url, user, password);

             System.out.println("连接成功");

         } catch (SQLException e) {

             System.out.println("连接失败：" + e.getMessage());

         }

     }

 }

 ```