It’s OK to think PHP as long as you are dealing with a web application. However, remember that your survival as you grow will depend on the robustness of your runtime. Facebook uses HipHop which they were compelled to build.
There are reasons why you will likely not be able to depend only PHP as your business grows. Here is a quote from a former Facebook engineer: “ Most of the backend services are written in C (programming language), C++ and Java. A lot of the C++ services rely on Boost (C++ libraries). Many of the internal tools and some glue code is written in Python (programming language). One of the services that underlies Chat, the channel servers (see http://www.facebook.com/note.php... ), is written in
Erlang (programming language). The backend services and the web frontend communicate using Apache Thrift. The iPhone app and iOS SDK are written in Objective-C (programming language).”