What exactly happens when you run `php example.com`? I wanted to answer this, so I decided to build my own PHP interpreter. Let me tell you what I learned and what does fancy words like opcodes and bytecode cache mean. With now two main rivals the standard PHP and HHVM from Facebook, there are plenty of choices to run your code fast. But they are both complicated projects, making it hard to understand the inner workings. Rather than relying on them to magically do the right thing, understand the principles of programming languages.