Need help to rewrite PHP function - php

Question

I have a function that parses text form posts and if there's a link in the post it'll redirect the link to a page that warns a users about external link before they click it.
function url2link($txt) {
$setUrl = preg_replace("/(http:\/\/|(www\.))(([^\s<]{4,68})[^\s<]*)/", '$2$4', $txt);
return $setUrl;
}
I need to modify this function by adding a check for domain in the link. If the link is from my own domain, just convert it into clickable link like this:
$setUrl = preg_replace("/(http:\/\/|(www\.))(([^\s<]{4,68})[^\s<]*)/", '$2$4', $txt);
but if it is a link to an external domain -- make a link point to a warning page (top example).
I am sort of stuck here because I have no idea how to add this check. There could be multiple links in a post, some may have local, some external links and some a mix.

Answer 1

Try with preg_replace_callback, then you can process the matches to decide whether it's your own domain or some other.

Answer 2

OK, as it turned out preg_replace_callback was exactly what I needed in this case. php.net documentation sucks. I found another article that made it clear what it is and how it works. I modified my function but it doesn't work... What am I missing?
function url2link($txt) {
$checkDomain = preg_replace_callback('/(http:\/\/|(www\.))(([^\s<]{4,68})[^\s<]*)/', 'linkDomain', $txt);
function linkDomain($matches) {
$host = parse_url($matches[0], PHP_URL_HOST);
$host = ltrim($host, 'www.');
if ($host == 'mydomain.com') {
$setUrl = preg_replace("/(http:\/\/|(www\.))(([^\s<]{4,68})[^\s<]*)/", '$2$4', $matches[0]);
} else {
$setUrl = preg_replace("/(http:\/\/|(www\.))(([^\s<]{4,68})[^\s<]*)/", '$2$4', $matches[0]);
}
}
return $setUrl;
}