Point 1: It depends upon your interest.
Point 2: Say if you go in for PHP, you can be a part of a web startup or else you can be part of Internet Giants like Google, Yahoo, Amazon, Ebay, Twitter etc. If you are a part of either then all the backend stuff involving Algorithms like search, recommendation engines, document clustering, distributed computing stuff would be done in either C++ or Java (Examples: Google Search (C++), Apache Solr/Hadoop (Java), Sphinx Search (C++)), the front end would/can be php. So now you can very well understand the importance of being a C++ developer as compared to PHP Developer. Again go to point 1 if it does not satisfy you. Also it depends on which company you are working for and type of work you are doing. If you are doing C++ in a service based company, probably you may be earning less as conmpared to if you are a PHP developer at Amazon or Ebay. It all depends. But to reiterate C++ is superior IMO.
Point 3: IMO learn php (learning curve would be very fast as you know C++) and then join an Internet Company where you can contritube to both sides of development i.e backend & frontend (you can get such stuff in a web startup). That will really be good for your career as you will come to know the complete system from front end to backend.