有许多不同的方法可以编写对象来处理数据库的连接和查询。
最重要的是你想达到什么目标。
我想这些会是你想拥有的一些功能。

• 单一连接
• 运行SQL并返回mysqli结果。
• 对转义值的连接的访问。
• 看起来您希望将凭据存储在对象本身中（我建议将它们作为__construct()中的值传入

这是一些基本的功能，可以很容易地扩展apon.

class databaseConnection {
    //private $hostname = 'hn.app.dev';
    //private $username = 'root';
    //private $password = '';
    //private $database = 'hn_app_dev';

    private $connection; // this is where the object will store the connection for other methods to access it
    
    public function __construct($host, $username, $password, $database)
    //public function connect() {
        $this->connection = mysqli_connect($host, $username, $password);
        if ($host) {
            $this->connection->select_db($database); 
            if (!$this->connection) {
                die('An error occured while trying to connect to the database.');
            }
            return $connection;
        }
    }

    // this is so databaseConnection $db can access the connection for escaping MySQLi SQL
    public function connection(){
         return $this->connection;
    }

    function fetch_from_db($query) {
        //$connection = new databaseConnection(); // I don't believe you really want to create a instance of this object inside of itself
        //$query = mysqli_query($connection->$connection, 'QUERY');
        $query = $this->connection->query($query); // we will use the object oriented style instead of the above procedural line
        return $query; // return the results to the $results var in the bottom example
    }

    // this is a magic function that is called when the object is destroyed, so we will close the connection to avoid to many connections
    function __destruct(){
      $this->connection()->close();
    }
}

// make a new datbaseConnection class with specific credentials
$db = new databaseConnection('localhost', 'someuser', 'somepass', 'somedb');
$sql = 'SELECT * FROM tableName LIMIT 100';

// call the fetch_from_db function from the new databaseConnection $db
$results = $db->fetch_from_db($sql);

// print the results
while($result = $results->fetch_assoc()){
  print_r($result); // print_r on each row selected from db
}

您可以在手册中学习更多关于OOP和PHP对象的知识，并且在线上有许多教程专门介绍管理数据库连接和查询的类。

• http://php.net/manual/en/language.oop5.php

希望这有帮助!

如果你想把它保存在一个类中，你永远不要调用connect()函数，但是如果你想在初始化类时连接它，把connect()函数改为__construct()，并删除return，把它赋给一个公共变量。

class databaseConnection {
    private $hostname = 'hn.app.dev';
    private $username = 'root';
    private $password = '';
    private $database = 'hn_app_dev';
    public $connection;
    public function __construct() {
        $host = mysqli_connect($this->hostname, $this->username, $this->password);
        if ($host) {
            $connection = mysqli_select_db($host, $this->database);
            if (!$connection) {
                die('An error occured while trying to connect to the database.');
            }
            $this->connection = $host;
        }
    }
}

之后，您可以在函数中获取数据库连接，如下所示：

function fetch_from_db($query) {
    $connection = new databaseConnection();
    $query = mysqli_query($connection->connection, 'QUERY');
}

现在，在说了所有这些之后，你不想在每个函数中创建一个新的示例来访问数据库，所以可以使它成为静态的，并创建一个init()函数，这样它在整个应用程序中占用更少的内存。
对于类文档，PHP的Classes/Objects页面会有帮助。特别是“示例”链接。

<?php

class databaseConnection {
    private $hostname = 'localhost';
    private $username = 'root';
    private $password = '';
    private $database = 'onlinylh_sggsfaculty';
    public function connect() {
        $host = mysqli_connect($this->hostname, $this->username, $this->password,$this->database);
        if ($host) {
            return $host;
        }
        else{
            echo "Error";
        }
    }
}
function fetch_from_db($query) {
    $conn = new databaseConnection();
    $r = mysqli_query($conn->connect(), $query);
}
fetch_from_db()
?>

这对我很有效。