PHP autoload: spl_autoload_register() and Composer

Autoloading loads a class's file at the moment the code first uses it: no more lines of require_once to line up, and no more loading order to respect.

spl_autoload_register(function (string $classe): void {
    $prefixe = 'Gazette\\';
    if (!str_starts_with($classe, $prefixe)) {
        return; // not a gazette class: another autoloader will take care of it
    }
    $fichier = __DIR__ . '/../src/' . str_replace('\\', '/', substr($classe, strlen($prefixe))) . '.php';
    if (is_file($fichier)) {
        require $fichier;
    }
});

$article = new Gazette\Article('The pumpkin festival'); // src/Article.php is loaded here

When PHP meets a class it doesn't know, it calls the functions registered with spl_autoload_register(), one by one, before throwing “Class not found”. Each one receives the full class name, namespace included, and loads the right file if it knows where to find it. So a class is only loaded if it is used, and a class added to src/ is available without declaring anything.

In a real project, you no longer write this function yourself: the “namespace prefix to folder” rule is called PSR-4, and Composer generates it in vendor/autoload.php, the only file you still load by hand. The classic traps: a file name that doesn't exactly match the class name (case matters on Linux, but not on macOS or Windows by default: code that works on your Mac breaks on the Linux server), and an autoload rule changed in composer.json without running composer dump-autoload again.