Add rel="nofollow" for all links with PHP?

function nofollow($html, $skip = null) {
    return preg_replace_callback(
        "#(<a[^>]+?)>#is", function ($mach) use ($skip) {
            return (
                !($skip && strpos($mach[1], $skip) !== false) &&
                strpos($mach[1], 'rel=') === false
            ) ? $mach[1] . ' rel="nofollow">' : $mach[0];
        },
        $html
    );
}

echo nofollow('<a href="http://www.yourdomain.com" rel="nofollow">your text</a>');
// will be same because it's already contains rel="nofollow" parameter

echo nofollow('<a href="http://www.yourdomain.com">your text</a>'); // ad
// add rel="nofollow" parameter to anchor

echo nofollow('<a href="http://www.yourdomain.com">your text</a>', 'yourdomain');
// skip this link as internall link