What is the difference between client-side and server-side development? Example with PHP

Hello everyone! I just started learning web development and I can't understand the difference between the client-side and server-side. It seems like both involve code, but they work differently. For example, I wrote a simple PHP script that calculates the sum of two numbers: ```php ``` If I open this file in a browser, I see the result. But if I write the same thing in JavaScript, I also see it. So what's the difference? I've heard that PHP runs on the server, and JavaScript runs in the browser. But what does that mean in practice? Why can't everything be done only on the client or only on the server? And how do I know which code should run where? Please explain in simple terms, with examples. I'm especially interested in how this works in PHP. Thanks in advance.