Composer Autoloader

Part of the Object Oriented Programming section of Coddy's PHP journey — lesson 13 of 91.

Writing your own autoloader works, but real PHP projects use Composer to handle autoloading automatically. Composer reads your project's configuration and generates an optimized autoloader that follows PSR-4 standards.

The configuration lives in a composer.json file at your project root. Here's how you define the autoloading rules:

{
    "autoload": {
        "psr-4": {
            "App\\": "src/"
        }
    }
}

This tells Composer that any class starting with App\ should be loaded from the src/ directory. After setting this up, Composer generates the autoloader files in a vendor/ directory.

To use Composer's autoloader, you only need one line at the top of your entry file:

<?php
require 'vendor/autoload.php';

use App\Models\User;
use App\Services\Logger;

$user = new User("Alice");
$logger = new Logger();

That single require statement handles all class loading for your entire project. You can define multiple namespace mappings if needed:

{
    "autoload": {
        "psr-4": {
            "App\\": "src/",
            "Tests\\": "tests/"
        }
    }
}

Key Point: Composer's autoloader eliminates manual file includes and ensures consistent class loading across your project. It's the standard approach used by virtually all modern PHP applications and frameworks.

Challenge

Easy

Let's build a small application that uses Composer's autoloading approach to organize classes across multiple files. You'll simulate how a real PHP project structures its code with PSR-4 namespaces and a central autoloader.

You'll create four files that work together:

src/Models/Product.php — Define a Product class in the App\Models namespace. It should have public $name and public $price properties, a constructor that accepts and sets both values, and a getInfo() method that returns "[name]: $[price]".
src/Models/Customer.php — Define a Customer class in the App\Models namespace. It should have a public $name property, a constructor that accepts and sets the name, and a greet() method that returns "Welcome, [name]!".
vendor/autoload.php — Create a simple autoloader using spl_autoload_register that mimics Composer's behavior. It should map the App\ namespace prefix to the src/ directory, converting namespace separators to directory separators and appending .php to load the correct file.
index.php — This is your entry point file. Require the autoloader from vendor/autoload.php with a single line, then use the use keyword to import both App\Models\Product and App\Models\Customer. Create a Product with name "Laptop" and price 999, and a Customer with name "Alice". Print the result of calling greet() on the customer and getInfo() on the product (each on its own line).

The key concept here is that your index.php only needs one require statement to load the autoloader — after that, all classes are loaded automatically when you use them. This is exactly how modern PHP frameworks and applications work with Composer.

Cheat sheet

Composer automates class loading by reading a composer.json configuration file and generating an optimized autoloader that follows PSR-4 standards.

Define autoloading rules in composer.json:

{
    "autoload": {
        "psr-4": {
            "App\\": "src/"
        }
    }
}

This maps the App\ namespace to the src/ directory. Composer generates autoloader files in a vendor/ directory.

Use Composer's autoloader with a single require statement:

<?php
require 'vendor/autoload.php';

use App\Models\User;
use App\Services\Logger;

$user = new User("Alice");
$logger = new Logger();

Define multiple namespace mappings if needed:

{
    "autoload": {
        "psr-4": {
            "App\\": "src/",
            "Tests\\": "tests/"
        }
    }
}

Composer's autoloader eliminates manual file includes and is the standard approach in modern PHP applications.

Try it yourself

<?php
// TODO: Require the autoloader from vendor/autoload.php

// TODO: Use the 'use' keyword to import App\Models\Product

// TODO: Use the 'use' keyword to import App\Models\Customer

// TODO: Create a Product with name "Laptop" and price 999

// TODO: Create a Customer with name "Alice"

// TODO: Print the customer's greet() result

// TODO: Print the product's getInfo() result
?>