Are you sure that the files and directories up to the location of the file were root:root 755? I seldom have code owned by the same ID as what is running it -- one more layer of the onion that a "baddie" needs to get through to modify stuff. If the web user need to write a file I will of

If changing the ownership to the same as the user the web server is running as solved your problem, I would suggest that the permissions on the file were not set to 755, or you have your web server configured to deny access to scripts not owned by the web server user.

Update:Struck out confusing typo, incomplete thought