Simplest possible autoloader for PHP classes. Can be used and for namespaces

autoloader.php

<?php

spl_autoload_register('autoloader');
function autoloader($className)
{
    if(strpos($className, '\\') !== false) {
        $className = str_replace('\\', DIRECTORY_SEPARATOR, $className);
    }
    
    if(strpos($className, '_') !== false) {
        $className = str_replace('_', DIRECTORY_SEPARATOR, $className);
    }
    
    require_once($className.'.php');
}