Many php-applications allow editting or downloading files through a directory listing style interface. The problem with most of these applications is that they don't adequately check the directories to which the user is attempting access and don't trap this as an error. If the application is located on the core drive the user may actually be able to download core system files and be aware of your system configuration -- or worse download your documents that you'd rather not let the world have access to download.

Enter a directory validation function, using simple PHP techniques this function allows validation of a user specified path via a download listing script. The function accepts a root path which the user must not go below (towards the root of the drive) and a user path that the user is attempting to access. If the user path does not go below the root path the function will return a boolean true, and if not a boolean false.