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Learn Functional Programming in PHP

Referential Transparency

Explore the concept of referential transparency in PHP functional programming. Understand how function calls can be replaced by their outputs without changing program behavior, enabling pure functions, higher-order functions, and introducing lazy evaluation concepts despite PHP's eager evaluation nature.

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Overview

A referentially transparent function is one in which the function call is interchangeable with the output of the function. This interchangeability shouldn’t affect a program’s side effects or side causes. Like immutability, referential transparency is associable with pure functions. In fact, the latter is a metric for measuring the former.

PHP
<?php
$insertUnderscores = fn (string $text): string => ( 
    implode('_', explode(' ', $text))
);
var_dump($insertUnderscores('hello world') === 'hello_world');  // evaluates to true

The lambda function above replaces spaces in a text with underscores. This function’s output, with a "hello world" input, is compared with the underscore-separated string "hello_world". The comparison yields a positive result, implying that this function call could’ve been replaced with the ...