How to determine if file is writable and deletable

I need to determine if a file is truly deletable by a php script, Deleting permissions seem to be 
the same as writing, they probably have the same criteria.

is_writable() seems to virtually useless in most cases. It doesn't take into account the directory 
ownership/permissions; which, best I can tell, is the real determining factor.

I've resorted to having to determine the directory's ownership and then compare the directory's 
rights with the owner's and then the other ['world'].

E.g., Assume my script was loaded with ftp, so it's ownership is the site-name, and I want the scrip 
to be able to determine if it can delete a file. Thus, the file in question must have its "other" 
permissions include write.

Surely, there must be an easier way.

Thanks, Al........