In recent years, the demand for functional programming language over object-oriented programming has increased. One of the core reasons is that functional programming (FP) has inherent parallelism. While OOP is used widely, functional programming is quite significantly making a mark in recent times. As a result, languages such as Erlang, Elixir, Clojure, Scala, and Haskell are the most popular functional programming languages for programmers. PHP is not on the list, as PHP is considered an imperative and object-oriented language. Though PHP has lots of support for functional programming, it is mainly used for OOP and imperative programming. The core essence of FP is the lambda calculus, which denotes a formal system in mathematical logic and computer science for expressing computation by way of variable binding and substitution. It is not a framework or a new concept. In fact, functional programming predates all other programming paradigms. It has been there for a long time and will be there in the future as well, as the world is demanding for more concurrent computation and faster processing languages. In this chapter, you will learn about functional programming with PHP and how to implement data structures with functional programming.