PHP Composer and Autoloading: PSR-4 Explained

What is Composer?

Composer is PHP's dependency manager. It downloads libraries, manages version conflicts, and handles autoloading automatically. No more manual require statements. Every modern PHP project uses it.

Essential Commands

# Install globally
curl -sS https://getcomposer.org/installer | php
sudo mv composer.phar /usr/local/bin/composer

# Create new project
composer init

# Add packages
composer require guzzlehttp/guzzle        # HTTP client
composer require phpmailer/phpmailer      # Email
composer require --dev phpunit/phpunit   # Dev only

# Install from lock file (use on servers — exact versions)
composer install --no-dev --optimize-autoloader

# Update packages (dev only)
composer update

# Regenerate autoloader after adding classes
composer dump-autoload

composer.json Explained

{
    "name": "ezycoders/my-app",
    "require": {
        "php": ">=8.1",
        "guzzlehttp/guzzle": "^7.0",
        "phpmailer/phpmailer": "^6.8"
    },
    "require-dev": {
        "phpunit/phpunit": "^10.0"
    },
    "autoload": {
        "psr-4": {
            "App\\": "src/"
        }
    },
    "autoload-dev": {
        "psr-4": {
            "Tests\\": "tests/"
        }
    }
}

PSR-4 Autoloading

<?php
// Directory structure:
// src/
//   Models/User.php     → namespace App\Models;  class User
//   Services/Mailer.php → namespace App\Services; class Mailer

// src/Models/User.php
namespace App\Models;

class User {
    public function __construct(
        public readonly int    $id,
        public readonly string $name
    ) {}
}

// index.php — ONE require, everything else autoloaded
require_once 'vendor/autoload.php';

use App\Models\User;
use App\Services\Mailer;

$user = new User(1, 'Rahul');  // No require needed!
echo $user->name;              // Rahul

Version Constraints

{
    "require": {
        "vendor/pkg": "1.2.3",      // EXACT version
        "vendor/pkg": "^1.2.3",     // >=1.2.3 <2.0.0
        "vendor/pkg": "~1.2.3",     // >=1.2.3 <1.3.0
        "vendor/pkg": ">=1.0 <2.0", // range
        "vendor/pkg": "dev-main"    // from git branch
    }
}