PHP

Autoloading classes with spl_autoload_register in PHP

Stop writing require lines: map class names to files and let PHP load them on demand.

Naoki « segfault »
systèmes & back-end · updated Jan 2025

This guide revisits and updates an original tutorial from noiretaya.com (log.noiretaya.com/191). The code has been refreshed for current versions.

Register an autoloader

spl_autoload_register() takes a callback that PHP calls whenever an unknown class is referenced. Map the class name to a file and require it.

spl_autoload_register(function ($class) {
    $file = __DIR__ . '/src/' . str_replace('\\', '/', $class) . '.php';
    if (is_file($file)) {
        require $file;
    }
});

With this in place, new App\Mailer\SmtpClient() loads src/App/Mailer/SmtpClient.php automatically — no manual includes.

PSR-4 in practice

This is exactly what Composer does. In real projects, declare it in composer.json and let Composer generate the autoloader.

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

composer dump-autoload

Rule of thumb: one class per file, file path mirrors the namespace. Hand-rolled autoloaders are great for learning; ship Composer's PSR-4 autoloader in production.