PHP works on many platforms, including Windows, Linux, BSD, Solaris, and Mac OS X to name just a few - this book attempts to cover PHP from an OS-agnostic point of view, which means I have tried hard to make sure you can understand and follow as much code as possible irrespective of what platform you use.
To make things easier to read, I have lumped Linux, BSD, Solaris, Mac OS, and other Unix-like OSs into "Unix" - this saves me typing and saves you reading, so everyone is a winner.
Throughout the book I refer to "shell prompts" and "command lines". This is the text entry system for your OS - for Windows NT/2000/XP users this can be found by clicking Start, then Run, and entering "cmd". Linux/BSD users running from the console are already there, but if you are using a GUI such as KDE or GNOME, you will need to launch xterm, Konsole, Gnome Terminal or whatever you use to enter text commands. Mac OS X users should have "Terminal" available for launching. I encourage use of the command line because it lets you use PHP in "interactive mode" - to type things, and see them happen immediately.