how can i set the permission for the folder "12" so that i can save a file in it?
How you do it is OS specific, but what you do is generally one of the following.
You can ensure that your web server runs under the same user id as the owner of that directory. You can also set up a group, give that directory group write access, and then ensure that the user id that your web server is running under is in the same group as the owner of that directory. (I typically use this approach.) Finally, you can give that directory world write access.
(You might also consider using an absolute path. Not that that will fix the permissions issue.)