Add rel nofollow to all external links in given content. $skip contains domain name

nofollow_to_external.php

public 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
        );
}