Inheritance

Quick Introduction

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

In this lesson you learn only inheritance — 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 Employee Management. After a user signs in, you need inheritance working correctly before storing data or showing a dashboard.

Without understanding Inheritance, 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

• Inheritance 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 (Employee Management) uses inheritance 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 Inheritance. 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);

namespace Payments;

abstract class PaymentGateway {
    public function __construct(protected readonly string $merchantId) {}

    abstract public function charge(float $amount, string $currency): string;

    protected function audit(string $ref): void {
        error_log("[{$this->merchantId}] charge ref={$ref}");
    }
}

final class StripeGateway extends PaymentGateway {
    public function charge(float $amount, string $currency): string {
        $ref = 'ch_' . bin2hex(random_bytes(8));
        $this->audit($ref);
        return $ref;
    }
}

echo (new StripeGateway('acct_123'))->charge(99.00, 'USD');

Line by line

1. — part of the inheritance example; run the file to see the result.
2. declare(strict_types=1); — turns on strict type checking for this file.
3. namespace Payments; — part of the inheritance example; run the file to see the result.
4. abstract class PaymentGateway { — part of the inheritance example; run the file to see the result.
5. public function __construct(protected readonly string $merchantId) {} — part of the inheritance example; run the file to see the result.

Authenticated

Real-World Example

In the Employee Management, inheritance 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: Inheritance exercise for Employee Management