How to Use PSR-4 Autoloading in PHP: Simple Guide

To use PSR-4 autoloading in PHP, define a namespace-to-directory mapping in your composer.json file under the autoload section, then run composer dump-autoload. This lets PHP automatically load classes based on their namespaces without manual require statements.

Syntax

The PSR-4 autoloading syntax is defined in the composer.json file under the autoload key. It maps a namespace prefix to a directory path where class files live.

• Namespace prefix: The root namespace your classes use.
• Directory path: The folder where the classes for that namespace are stored.

Composer uses this mapping to find and load classes automatically.

json

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

Example

This example shows how to set up PSR-4 autoloading with Composer, create a class in the mapped namespace, and use it without manual includes.

php

// composer.json
{
  "autoload": {
    "psr-4": {
      "MyApp\\": "src/"
    }
  }
}

// src/Greeting.php
<?php
namespace MyApp;

class Greeting {
    public function sayHello(): string {
        return "Hello, PSR-4!";
    }
}

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

use MyApp\Greeting;

$greet = new Greeting();
echo $greet->sayHello();

Output

Hello, PSR-4!

Common Pitfalls

Common mistakes when using PSR-4 autoloading include:

• Not running composer dump-autoload after changing composer.json.
• Incorrect namespace or directory mapping causing classes not to load.
• Class files not matching the namespace and class name structure (file path must match namespace and class name).
• Forgetting to include Composer's vendor/autoload.php file.

php

// Wrong: Namespace does not match directory structure
// src/Greeting.php
<?php
namespace WrongNamespace;

class Greeting {}

// Right: Namespace matches directory
// src/Greeting.php
<?php
namespace MyApp;

class Greeting {}

Quick Reference

Tips for using PSR-4 autoloading:

• Namespace prefixes end with a backslash \\.
• Directory paths are relative to composer.json.
• Class files must be named exactly as the class with .php extension.
• Run composer dump-autoload after changes.
• Always include vendor/autoload.php in your entry script.

Key Takeaways

Define namespace-to-directory mapping in composer.json under autoload > psr-4.

Class file paths must match their namespaces exactly for autoloading to work.

Run composer dump-autoload after editing composer.json to update autoload files.

Include vendor/autoload.php in your PHP scripts to enable autoloading.

PSR-4 autoloading removes the need for manual require/include statements.