Woody Gilk has posted an article to his site looking at immutable data structures in PHP and a library that's come from the research and work he did to implement them in PHP.
As someone who most often works with PHP I often find myself envious of the more advanced data structures that are present in a language like Python. As an experiment, I decided to see if it would be possible to bring some of those basic structures to PHP while also preserving immutability. The result of this experiment is Destrukt.
He starts off talking about immutable data structures and introducing some of the basic concepts and usage around them. He notes that they "cannot be modified by accident" and how, if they do need to be changed, they'd actually be reassigned not updated. He then talks about PHP arrays, how they're normally used in PHP and how their flexibility can lead to potential issues in the code. His library implements more strict versions of the same functionality in the form of dictionaries, orderedlists, unorderedlists and sets. He includes examples of using the library to create these objects and how to get the data back out of them for evaluation.