Autoloading in PHP (spl_autoload_register & Composer)

Why autoloading

In a real project you may have dozens of class files. Writing a require line for each one is tedious and easy to get wrong. Autoloading lets PHP find and include a class file automatically the first time you use that class — so you write the loading logic once, not once per file.

The old way: many requires

Without autoloading, every file has to be included by hand:

<?php
  require "User.php";
  require "Product.php";
  require "Order.php";
  // ... and so on for every class
?>

This grows unmanageable fast, and forgetting one line causes a "class not found" error. Autoloading solves exactly this.

spl_autoload_register

Register one function that turns a class name into a file path and includes it.

<?php
  spl_autoload_register(function ($class) {
      require __DIR__ . "/classes/" . $class . ".php";
  });

  // No require for User needed — it loads automatically:
  $u = new User();
?>

Now the first time PHP sees new User(), it calls your function, which includes classes/User.php. Add more classes to the folder and they just work.

Composer autoload (PSR-4)

In modern PHP, Composer handles autoloading for you using the PSR-4 standard. In composer.json:

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

After running composer dump-autoload, you include the autoloader once:

<?php
  require "vendor/autoload.php";
  use App\Models\User;

  $u = new User();   // loaded automatically from src/Models/User.php
?>

PSR-4 maps the App\ namespace to the src/ folder, so App\Models\User is found at src/Models/User.php — no manual requires at all.

Common mistakes

• File name not matching class name. The autoloader builds the path from the class name — they must match exactly, including case.
• Wrong folder in the path. If the class is in a different folder than your autoloader expects, it will not be found.
• Forgetting composer dump-autoload. After adding new namespaced classes, regenerate the autoloader.
• Namespace not matching PSR-4 mapping. The namespace prefix must line up with the mapped folder.

Summary

• Autoloading loads class files automatically on first use.
• It replaces many require lines with one setup.
• spl_autoload_register is the built-in way without Composer.
• Composer + PSR-4 maps a namespace to a folder and autoloads everything.
• Class name and file name (and namespace and folder) must match.

Next, organise classes with namespaces, or structure a whole app with the MVC pattern.