Understanding PHP 8 expanded variance support
The concept of variance is at the heart of OOP. Variance is an umbrella term that covers how the various subtypes interrelate. Some 20 years ago, a pair of early computer scientists, Wing and Liskov, devised an important theorem that is at the heart of OOP subtypes, now known as the Liskov Substitution Principle.
Without going into the precise mathematics, this principle can be paraphrased as follows:
Class X can be considered a subtype of class Y if you are able to substitute an instance of X in place of an instance of Y, and the application's behavior does not change in any way.
Tip
The actual paper that first described and provided the precise mathematical formulaic definition of the Liskov Substitution Principle can be found here: A behavioral notion of subtyping, ACM Transactions on Programming Languages and Systems, by B. Liskov and J. Wing, November 1994 (https://dl.acm.org/doi/10.1145/197320.197383).
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