php中封装mysql类

<?php
class MySQLDB {
    private $host;
    private $username;
    private $password;
    private $dbname;
    private $conn;

    public function __construct($host, $username, $password, $dbname) {
        $this->host = $host;
        $this->username = $username;
        $this->password = $password;
        $this->dbname = $dbname;
        $this->connect();
    }

    private function connect() {
        $this->conn = new mysqli($this->host, $this->username, $this->password, $this->dbname);
        if ($this->conn->connect_error) {
            die("连接失败: " . $this->conn->connect_error);
        }
    }

    public function query($sql) {
        return $this->conn->query($sql);
    }

    public function fetch_assoc($result) {
        return $result->fetch_assoc();
    }

    public function close() {
        $this->conn->close();
    }
}

// 使用示例
$db = new MySQLDB('localhost', 'root', 'password', 'testdb');
$result = $db->query("SELECT * FROM users");
while ($row = $db->fetch_assoc($result)) {
    echo $row['username'] . "<br>";
}
$db->close();
?>

$stmt = $this->conn->prepare("SELECT * FROM users WHERE username = ?");
$stmt->bind_param("s", $username);
$username = $_POST['username'];
$stmt->execute();
$result = $stmt->get_result();