Want to build software that is more testable, easier to modify, and has fewer lines of code? Architecting with more immutable objects that are always in a valid state is the most important lesson I have learned in building better software applications. Using immutable value objects will lead to less checking, fewer bugs, and more DRY code, and will help avoid the “spooky action at a distance” problem in PHP. We will also learn how to use immutable objects and immutable collections to improve the design of our mutable entities. Lastly, we’ll see how immutable objects and functional programming can reduce complexity.