Use composer on comandline
Either one can only install Composer on Windows globally (most likely as it looks in this case) or you installed Composer on Windows globally. Yeah, in this case you must drop the
.phar particles from the command. It is possible to install Composer on e.g. on Linux globally too but the installation manual is indended too which is probably the best way if you have more than one wiki on your machine. The docu is indended to be as basic as possible. That's why I did not want to confuse people with even more steps.
Good anyway that you pointed this out for others.