PSR-4 directory mapping in PHP

Introduction

PSR-4 directory mapping helps PHP find your classes automatically by linking namespaces to folders. This makes your code organized and easy to load.

When you want to organize your PHP classes in folders that match their namespaces.

When you use Composer to autoload your PHP classes without manually including files.

When you build a PHP project with many classes and want to keep code clean and easy to maintain.

Syntax

PHP

"psr-4": {
    "Namespace\\": "path/to/folder/"
}

The left side is the namespace prefix with double backslashes escaped.

The right side is the folder path where classes for that namespace live.

Examples

This maps the namespace App\ to the src/ folder.

PHP

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

This maps the namespace MyProject\Utils\ to the lib/utils/ folder.

PHP

"psr-4": {
    "MyProject\\Utils\\": "lib/utils/"
}

Sample Program

This example shows a class Hello inside the App namespace mapped to the src/ folder. Composer autoloads the class automatically.

PHP

<?php
// File: src/Hello.php
namespace App;

class Hello {
    public function say() {
        return "Hello from App\\Hello!";
    }
}

// File: test.php
require 'vendor/autoload.php';

use App\Hello;

$greet = new Hello();
echo $greet->say();

Important Notes

Make sure the folder path ends with a slash /.

Namespace and folder structure must match exactly for autoloading to work.

Composer's autoload section in composer.json is where you set PSR-4 mapping.

Summary

PSR-4 maps namespaces to folders for automatic class loading.

It keeps your PHP code organized and easy to maintain.

Use Composer to set up and use PSR-4 autoloading in your projects.