To many beginners the subject of variables is usually pretty scary, and often a reasonably difficult concept to grasp, the reason for this however is usually because most modern languages require some kind of indication as to what type of data a variable will hold, this in turn often confuses beginners because they don't know what type of data relates to what kind of type.

They describe variables (using sample assignments like strings and numbers) and talk some about scope and how it affects their visibility. There's also a brief mention of the superglobals there close to the end.

When your code assigns to a parameter in a function/method, use a temporary variable instead. [...] It will make your code much more readable and prevents by-reference confusion and therefore big problems in the future.

His example code uses the illustration of calling a price() method in a class to modify the inputVal value based on other inputted information. His suggestion is to not work with the actual inputVal value passed in (so as to avoid issues if it happens to be passed my reference later), but to work with a temporary variable - $result - inside the method.