In the last article we learnt how to create an immutable data structure in PHP. There were a few issues to work through, but we got there in the end. Now onto making the immutable class more useful and easier to create modified copies. Note that these are copies and not modifications, in-place, to the original objects.
He then moves on from the "simple" mutation method previously used (making a new immutable object when a property changes). When the property list starts getting more complex simple single value references no longer scale. He makes use of methods internal to the class to modify the values and return a new immutable instance with the updated value. He shows how to modify this to prevent the setting of unexpected properties and how to expand it out to allow the input of an array of values to update and how to handle required/optional property values.