Re: Is the xdebug's non-free license necessary?
The trouble, I think, is that "derived product" has a legal meaning
(in the context of copyright) contrary to your common-sense
interpretation. Anything other than an exact copy of the source code
you distribute (or, if you distribute binaries, exact copies of them)
-- even an unpatched but independently compiled binary -- is a
"derived product" in this sense.
There's a legal instrument for control over other people's use of a
name -- it's called a trademark. It is governed by different law and
the criteria for infringement are different (e. g., "tarnishment" for
use of your product's name in a way that devalues the trademark, such
as by using it to refer to a shoddy derived product). For instance,
Linus Torvalds has registered "Linux" as a trademark precisely to deal
with this concern. The legal entities behind Apache and PHP have
probably also registered these trademarks and act as necessary to
defend them (necessary in order to retain ownership of the trademark
-- again different from copyright law).
When you impose non-exact naming constraints in your license, I think
you are implicitly dragging in additional trademark considerations,
and that complicates the interpretation of the license as free or
non-free. I don't think anyone is hostile to your intention, but
there's a history of dispute over the details. And in any case,
adding that clause doesn't give you any legal recourse that you didn't
already have under the legal definition of a "confusingly similar"
trademark. (I think -- IANAL, and I'm going on US/California law.)
(I hope I'm not insulting your intelligence -- it was explained to me
in similar terms not so long ago.)