Most programmers know at least a little about c++. If you want to become a programmer then it's likely that you will have to learn it in your education itself so it's gonna help to know a little c++ ;)
C++ is more general purpose than Php. But Php is better for website development.
FWIW I write a TON of what I would call disposable programs that do things for me that would have taken too long any other way. Most are less than 1 page of code, a few min of work. c++ can generate php code, and often does on the fly at the server side. Its worth knowing how to do that, but its up to you. You don't have to master the language to be able to do quite a bit...