Before we go further with CMS development, let's look at a problem that can be neatly solved using PHP5. Substantial systems do not consist of a single file of code. Whatever our exact design, a large system should be broken down into smaller elements, and it makes sense to keep them in separate files, if the language supports it. Code is more manageable this way, and systems can be made more efficient.

As we are considering only PHP implementations, the source code files are used at runtime. PHP is an interpreted language and, at least in principle, runs the actual source code. So we need a good technique for handling many source files at runtime.

This creates issues; a paramount one is security. Another is ease of coding, where it is tedious and cumbersome to have to repeatedly include code to load other files. Yet another is efficiency, as we do not want to load code that is not needed for a particular request.