My first attempt at a PHP autoloader. Is based on the one in PSR-0, but with the added check of constraining it to just the project's namespace which is the polite thing to do since spl_autoload functions are stackable

autoload.php

spl_autoload_register(function($className){
    $projectNamespace = "PHPWebDriver";

    $className = ltrim($className, '\\');
    $fileName  = '';
    $namespace = '';
    if ($lastNsPos = strrpos($className, '\\')) {
        $namespace = substr($className, 0, $lastNsPos);
        $className = substr($className, $lastNsPos + 1);
        $fileName  = str_replace('\\', DIRECTORY_SEPARATOR, $namespace) . DIRECTORY_SEPARATOR;
    }
    $fileName .= str_replace('_', DIRECTORY_SEPARATOR, $className) . '.php';

    if (substr($namespace, 0, strlen($projectNamespace)) === $projectNamespace && file_exists($fileName)) {
        require $fileName;
    }
});