fetch_field_direct()函数用于获取当前结果集中的字段信息，包括字段名、类型、长度等信息。
该函数的参数如下：
用法示例：
$result = $mysqli->query("SELECT * FROM users");
$field_info = $result->fetch_field_direct(0);
echo "Field name: " . $field_info->name;
echo "Field type: " . $field_info->type;
echo "Field length: " . $field_info->length;
