In another part of his "Composing Methods" series, Tim Koschuetzki posts about removing assignments to parameters today - working with a temporary variable inside a method rather than the actual passed in value.
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.