Show disk usage, file sizes, how large (in bytes) each directory is, etc.
The Observer design pattern is very common, and luckily is a very easy concept to understand. Here it is explained!
The essential keyboard shortcuts that you need to be aware of when using PHPStorm
The front controller is a simple to understand design pattern where you have a main controller that handles every request for a website. It is a commonly used design pattern for many MVC based web applications.
An explanation of the S.O.L.I.D. principles, and how to use them in your PHP scripts.