I apologize in advance, because this might not be the right place to ask this question.
I am trying to develop a WebApp which will use PHP/MySQL in the back-end (that's because I know only PHP better than other back-end languages). At this point I can't learn another new language. The app will be initially running on Linux (Ubuntu 10.04 on VMware ESXi 5). In future the users and database will grow higher if the App does well. It may have thousands of users and a huge number file-uploads and database-records. In that case, I need to scale my App. Like what Facebook is doing right now. They are porting and optimizing their old scripts to C++ and Java and runs them directly so that it gets better performance.
Do you have any suggestion for me? I didn't start building the App yet. How should I plan on designing the architecture for the App concerning the scaling issue? I will really appreciate if you could give me any idea on this, so that I may start making plans for it before jumping on the actual coding.