Dependency Resolution Composer

How Composer Maps Files → Classes → Namespaces

Composer does not “scan files randomly”. It uses autoload rules (usually PSR-4) to map:

Namespace → Folder path → PHP file → Class name

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📦 PSR-4 Autoloading Rule

In composer.json:

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

👉 Meaning:

• App\ → root namespace
• src/ → folder where code lives

So:

App\Foo\Bar → src/Foo/Bar.php

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The Core Mapping Rule

1. Namespace = folder structure

Namespace: App\Service\UserService

Becomes:

src/Service/UserService.php

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2. Class name must match file name

Inside file:

<?php
 
namespace App\Service;
 
class UserService
{}

👉 File MUST be:

UserService.php

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3. Full resolution rule

Composer resolves like this:

App\X\Y\Z
↓
src/X/Y/Z.php
↓
class Z inside file

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Concrete Example

composer.json

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

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File structure

project/
 ├── src/
 │    └── Controller/
 │         └── HomeController.php
 ├── vendor/
 └── composer.json

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File content

<?php
 
namespace App\Controller;
 
class HomeController
{
    public function index()
    {
        return "Hello";
    }
}

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Usage in code

require __DIR__ . '/vendor/autoload.php';
 
use App\Controller\HomeController;
 
$controller = new HomeController();
echo $controller->index();

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⚠️ Common mistakes

❌ Namespace mismatch

namespace App\Controllers; // wrong folder expectation

But file is:

App/Controller/HomeController.php

👉 This breaks autoloading.

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❌ Wrong file name

class HomeController → Home.php ❌

Composer won’t find it.

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❌ Forgetting dump-autoload

After changing autoload config:

composer dump-autoload

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When Composer “knows” your classes

Composer builds a map inside:

vendor/composer/autoload_psr4.php

This is why vendor/autoload.php can resolve everything instantly.

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Mental model (important)

Think of Composer autoload like:

“A router for PHP classes”

Namespace → Route
Folder → Controller group
File → Endpoint handler
Class → Functionality