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........