PHP - roots list

Unfortunately, I don't know another way how to resolve all roots (drives) under Windows:

public function getRoots() {
    $roots = array();
    for ($i='A'; $i<'Z'; $i++) {
        $disk=$i.':\\';
        if (is_dir($disk) && is_readable($disk))
        $roots[] = $disk;
    }
    return $roots;
}

Somebody know?