在mysqli预处理语句中使用null

$conn = new mysqli($servername, $username, $password, $dbname);
$stmt = $conn->prepare("INSERT INTO mytable (col1, col2, col3) VALUES (?, ?, ?)");

// 绑定参数
$stmt->bind_param("sis", $value1, $value2, $value3);

// 设置参数值
$value1 = "Some value";
$value2 = NULL;
$value3 = 123;

// 执行预处理语句
$stmt->execute();

// 关闭预处理语句和连接
$stmt->close();
$conn->close();