function canonical_path($path) {
    $canonical = preg_replace('|/\.?(?=/)|','',$path);
    while (($collapsed = preg_replace('|/[^/]+/\.\./|','/',$canonical,1)) !== $canonical) {
        $canonical = $collapsed;
    }
    $canonical = preg_replace('|^/\.\./|','/',$canonical);
    return $canonical;
}