php mysql连接类

<?php
class MySQLConnection {
    private static $instance;
    private $conn;

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

    public static function getInstance() {
        if (self::$instance == null) {
            self::$instance = new MySQLConnection();
        }
        return self::$instance;
    }

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

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

// 使用示例
$mysql = MySQLConnection::getInstance();
$result = $mysql->query("SELECT * FROM users");
while ($row = $result->fetch_assoc()) {
    echo $row['username'] . "<br>";
}
$mysql->close();
?>