Chapter 1, Functions as First Class Citizen in PHP, discusses how functional programming, as the name suggests, revolves around functions. In this chapter, you will learn the about the various way they can be declared and used in PHP.

Chapter 2, Pure Functions, Referential Transparency, and Immutability, covers the three concepts that are the cornerstone of any functional code base. You will learn what they are about and how to apply them to our benefit.

Chapter 3, Functional Basis in PHP, discusses how functional programming, like any paradigm, rests upon a few core concepts. This chapter will present them in a simple fashion before going further.

Chapter 4, Composing Functions, describes how functions are often used as a building block using function composition. In this chapter, you will learn how to do it in PHP what it is important to keep in mind when doing so.

Chapter 5, Functors, Applicatives, and Monads, starts with easier concepts, such as the functor and the applicative, and we will build up our knowledge to finally present the monad in a light that should dispel some of the fear floating around this term.

Chapter 6, Real-life Monads, helps you learn about some real-life usage of the monad abstraction and how it can be used to write better code.

Chapter 7, Functional Techniques and Topics, brushes upon topics such as type systems, pattern matching, point-free style, and others from the vast field of functional programming.

Chapter 8, Testing, teaches you that functional programming not only helps with writing code that is easier to understand and maintain, but it is also great to facilitate testing.

Chapter 9, Performance Efficiency, lets you know that using functional techniques in PHP has a cost. We will first discuss it and then see how it can help in other performance-related topics.

Chapter 10, PHP Frameworks and FP, introduces a technique that can be applied to improve your code in any project, as there is currently now dedicated framework for functional programming in PHP.

Chapter 11, Designing a Functional Application, will present you with some advice if you want to develop a whole application using the most functional code possible. You will also learn about Functional Reactive Programing and the RxPHP library.

Appendix, What are We Talking about When We Talk about Functional Programming?, is a presentation and history of functional programming alongwith its benefits and a glossary. It’s really the first part of the book you should read, but as we don’t approach the subject from the PHP angle, it is presented as an appendix.

You will need to have access to a computer with PHP installed. It will be easier if you know how to use the command line, but all examples should also work in a browser with maybe some small adaptations.

While learning functional programming, I also recommend the usage of a Read-Eval-Print-Loop (REPL). I personally used Boris when writing this book. You can find it at https://github.com/borisrepl/boris. Another great option is PsySH (http://psysh.org).

Although not at all mandatory, a REPL will allow you to quickly test your ideas and play around with the various concepts that will be presented in this book without having to juggle between your editor and command line.

I also assume you have Composer available and that you know how to use it to install new packages; if not, you can find it at https://getcomposer.org. Multiple libraries will be presented throughout the book and the preferred way to install them is using composer.

All the code written in the book was tested on PHP 7.0, which is the de facto recommended version. It should, however, also run on any newer version. Running most of the examples should also be fine on PHP 5.6 after making some minor adaptations. We will use the new scalar type hinting feature introduced in PHP 7.0 throughout the book, but if you remove those, the code should be readily compatible with lower versions.

This book requires no knowledge of functional programming; prior programming experience is, however, required. Also, basic concepts from object-oriented programming will not be covered in depth.

Deep knowledge of the PHP language is not mandatory, as uncommon syntax will be explained. The book should be understandable to someone who hasn't written a single line of PHP code, with some effort.

This book can be considered as a beginner book about functional programming in PHP, meaning that we will build knowledge incrementally. However, the topic being pretty vast and the limited page count, we will move quickly at times. This is why I encourage you to play with the variously presented concepts as we learn them and take some time at the end of each chapter to make sure you understood it correctly.

In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning.

Code words in the text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles are shown as follows: "The next lines of code read the link and assign it to the BeautifulSoup function."

A block of code is set as follows:

<?php
function getPrices(array $products) {
  $prices = [];
  foreach($products as $p) {
    if($p->stock > 0) {
      $prices[] = $p->price;
    }
  }
  return $prices;
}

When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:

<?php
function getPrices(array $products) {
  $prices = [];
  foreach($products as $p) {
    if($p->stock > 0) {
      $prices[] = $p->price;
    }
  }
  return $prices;
}

Any command-line input or output is written as follows:

composer require rx/stream

New terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "Clicking the Next button moves you to the next screen."

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