Quick&dirty PSR-0 implementation (tested)

psr-0

spl_autoload_register(function ($class)
{
    $file = __DIR__.DIRECTORY_SEPARATOR.'classes'.DIRECTORY_SEPARATOR;
    
    if ($class[0] == '\\') {
        $class = ltrim($class, '\\');
    }
    
    if (false !== ($pos = strrpos($class, '\\'))) {
        $file .= str_replace(
            '\\',
            DIRECTORY_SEPARATOR,
            substr($class, 0, ++$pos)
        );
    }
    
    $file .= str_replace(
        '_',
        DIRECTORY_SEPARATOR,
        $pos ? substr($class, $pos) : $class
    ).'.php';
    
    if (file_exists($file)) {
        require $file;
    }
});