I found it most useful to follow what people are already used to, which is to say that it follows the principles laid out by Symfony. The code for the framewor

I found it most useful to follow what people are already used to, which is to say that it follows the principles laid out by Symfony. The code for the framework is its own composer library which is pulled in to the vendor directory, the code for the application sits within src/{BundleNamespace}. If there's a service, it's a service, if there's a controller, it's in Controller, etc. The aim is to maximise ergonomics and familiarity while minimising surprises. If a developer can pick up your microframework and be instantly writing code then you've done your job right.