Since you already know C/C++ I'm willing to wager you are able to pick up Python or Ruby in a few days if not less. Both have frameworks quite well suited to web programming (Django or one of the other WSGI frameworks for Python) and obviously Rails for Ruby. Personally I prefer Python's syntax and community, but other then that the differences with Ruby are fairly trivial.

I would recommend against going with PHP (especially) since you say you already have coding experience. As such the usual "PHP is easier to learn" argument hardly holds and I think the more consistent style of libraries is more pleasant for experienced programmers.