When I first learned to program, I made many objects that were mutable. I made lots of getters and lots of setters. I could create objects using a constructor and mutate and morph the heck out of that object in all kinds of ways. Unfortunately, this led to many problems. My code was harder to test, it was harder to reason about, and my classes became chock full of checks to ensure that it was in a consistent state anytime anything changed. ... Now, I favor creating immutable objects.

http://blog.joefallon.net/2015/08/immutable-objects-in-php/

The post clearly explains the benefits of using immutable objects and a nice example.