PHP 8: Constructor Property Promotion

This content originally appeared on DEV Community and was authored by Ibrahim Alausa

Introduction

If you have been a software engineer for a while, you know that software engineers love using scary names to describe simple things. The term constructor property promotion should not scare you because you might be using it in your codebase already without realizing it has a fancy name.

Here are the things you will achieve after reading this article.

You will be able to refactor a PHP class to use constructor property promotion instead of using the default syntax used when setting class properties in the constructor function.
You will know the different ways of accessing class properties after setting them using the constructor property promotion syntax.
You will know the common mistakes that lead to errors when using the constructor property promotion syntax in your PHP class and how to avoid them.

Interested? Let's move 🚀

Prerequisites

A basic understanding of Object-Oriented Programming in PHP
You must have PHP 8 installed to enjoy this feature in your code or project. Alternatively, you can test this feature on phpsandbox by selecting PHP8 as your preferred version

What is Constructor Property Promotion? 🤔

Constructor property promotion is a feature introduced in PHP 8 that allows values passed to a constructor to be automatically set as class properties without the need to write the assignment statement in the body of the constructor.
For a better understanding of the concept, let's take a look at the way values were assigned to class properties before the introduction of the constructor property promotion feature in PHP8.

Before constructor property promotion 🥱

Before constructor property promotion was introduced, here’s how you will typically set the
firstName and lastName properties passed to the Employee class through the constructor.

<?php

class Employee
{

  protected $firstName, $lastName;

  public function __construct(string $firstName, string $lastName)
  {
    $this->firstName = $firstName;
    $this->lastName = $lastName;
  }
}


$employee = new Employee('Ibrahim', 'Alausa');

In the code above, we are declaring the properties firstName and lastName without any initial value. Then, in the body of the constructor, we set the firstName and lastName properties to the value passed to the constructor. In this example, that is **Ibrahim **as the value of the firstName property and **Alausa **as the value of the lastName property.

The new way for the cool programmers 😎

Now, let's rewrite the Employee class to use constructor property promotion. Let's see what our code will look like in this case

<?php

class Employee
{
  public function __construct(protected string $firstName, protected string $lastName)
  {
  }
}


$employee = new Employee('Ibrahim', 'Alausa');

That's it. Yes. Isn't this beautiful? Just in case your answer is *NO * well, that was a rhetorical question, my friend. It is beautiful.

What exactly is different

By adding an access modifier (protected, private, etc) to each parameter in the constructor, PHP understands that you are trying to “promote” the constructor parameter to a class property.

Important points to note when working with constructor property promotion

1. Accessing class properties 🔐

Within the constructor, promoted properties can be accessed with or without the $this keyword since they are still within the function scope where they were declared. Let's see what our code will look like in this case

<?php

class Employee
{
  public function __construct(protected string $firstName, protected string $lastName)
  {
    var_dump($firstName);  Ibrahim ✔️
    var_dump($this->firstName); Ibrahim ✔️
  }
}


$employee = new Employee('Ibrahim', 'Alausa');