I have found that if you know that much languages, you don't have to learn a new language, you just simply start coding in it. And if you don't know something you Google it.
The C/C++ syntax is not far from Java and PHP, so it should not be that difficult.
Another thing: Don't just diversify for the fun of it. Certain languages are better than others for specific tasks. If you learn stuff you won't use its a waste of energy (not that I think C++ is bad or anything). Therefore if you you need to code something that would be best in C/C++, then learn it - else don't. If you don't know the pros and cons about languages, Google will have plenty to say.