PHP is traditionally only used on the internet to power our websites, but it's also capable of doing more. Since version 4.3 PHP ships with a special version which can be used to execute scripts on the command line, and do certain system tasks. If you're a Linux user, you probably know what the command line is, but if you're a Windows user you might not.
They show the code/commands in the examples they give in the context of it running on a Windows machine, but it's not a far leap to work with it on a unix-based system as well. They start with the basics - how to get to it, how to use it - before moving on to other features, like taking in arguments. They wrap it up with a sample application, a calculator, that can do the four basic operations on the two numbers entered.