确定PHP应用程序的服务器要求[关闭]

I'm currently working on a web app and if all goes to plan, it could very well grow extremely quickly. This is the first app I'm building where server requirements are a big question to me. What can I do to determine how much bandwidth, ram, and processing power I'm going to need?

Is there a specific term to what I'm trying to find out here? I've heard of stress testing but I'm not sure that's what this is.

Any help would be great.