rsync, for one, disagrees

rsync man

this ok:

A trailing slash on the source changes this behavior to avoid creating an additional directory level at the destination. You can think of a trailing / on a source as meaning “copy the contents of this directory” as opposed to “copy the directory by name”

But here we see that leading '/' sometimes does not mean root

if the pattern starts with a / then it is matched against the start of the filename, otherwise it is matched against the end of the filename.

Thus “/foo” would match a file called “foo” at the transfer-root (see above for how this is different from the filesystem-root).

I think filename is filename. It points to file. When you passing arguments to command line programs - it's different thing - sometimes you need special syntax, depending on what you want to do

"mv foo bar/" is only the same as "mv foo bar" if you are 100% sure that bar already exists and is a directory.

yep, but there are problems when it's symlink to a directory example

Also basename

In the usual case, dirname() returns the string up to, but not including, the final '/'