在函数全局变量中使用mysqli不起作用

global $mysqli;
$mysqli = new mysqli("localhost", "username", "password", "database");

function myFunction($mysqli) {
    // 在函数中使用$mysqli对象
}

$mysqli = new mysqli("localhost", "username", "password", "database");
myFunction($mysqli);

class Database {
    private $mysqli;

    public function __construct() {
        $this->mysqli = new mysqli("localhost", "username", "password", "database");
    }

    public function myFunction() {
        // 在函数中使用$this->mysqli对象
    }
}

$database = new Database();
$database->myFunction();