Readonly Properties

Quick Introduction

Readonly Properties is a single PHP idea you'll use in almost every backend project.

In this lesson you learn only readonly properties — not five topics at once. By the end you can write a small working example and explain it in an interview.

We connect each lesson to our course projects: Login System, Blog CMS, REST API, Inventory, Employee Management, and E-Commerce Backend.

Business Problem

You're building the REST API. After a user signs in, you need readonly properties working correctly before storing data or showing a dashboard.

Without understanding Readonly Properties, the team ships bugs: wrong totals, broken sessions, or type errors that only appear in production. This lesson fixes that with one clear pattern you can copy into your project today.

Core Concept

• Readonly Properties is one focused idea — learn it before mixing with other PHP topics.
• Use it in PHP 8.3+ with declare(strict_types=1); at the top of every file.
• Our course project (REST API) uses readonly properties in real handlers.
• Run small scripts with php file.php after each change — don't just read.
• Interviewers ask for a one-minute explanation plus a tiny code sample.

Syntax

Core syntax for Readonly Properties. Every keyword below appears in production PHP — Laravel and Symfony use the same primitives under the hood.

declare(strict_types=1);

Keywords: declare · strict_types · namespace

Step-by-Step Example

Run this script locally. Change one value, run again, and watch what changes.

<?php
declare(strict_types=1);

readonly class Address {
    public function __construct(
        public string $line1,
        public string $city,
        public string $postalCode,
        public string $country = 'US',
    ) {}

    public function formatted(): string {
        return "{$this->line1}, {$this->city} {$this->postalCode}, {$this->country}";
    }
}

readonly class Customer {
    public function __construct(
        public string $id,
        public string $email,
        public Address $billingAddress,
    ) {}
}

$c = new Customer('c1', 'a@b.com', new Address('1 Main St', 'Austin', '78701'));
echo $c->billingAddress->formatted();

Line by line

1. — part of the readonly properties example; run the file to see the result.
2. declare(strict_types=1); — turns on strict type checking for this file.
3. readonly class Address { — part of the readonly properties example; run the file to see the result.
4. public function __construct( — part of the readonly properties example; run the file to see the result.
5. public string $line1, — part of the readonly properties example; run the file to see the result.

Readonly Properties OK

Real-World Example

In the REST API, readonly properties appears in a single request handler — not spread across ten files. Keep the example small, test it with php, then paste the pattern into your project branch.

<?php
declare(strict_types=1);

// TODO: Readonly Properties exercise for REST API