PHP filters website links in HTML. PHP implements domain name whitelist function.

/**
 * Filter external site links
 * @param array $local_domain Domain name array of this site
 * @param string $message text content
*/
function replace_outer_links($local_domain_arr, $message) {
$pattern= '/<[^>]*href=['"]http[s]?://(?!' ;
$i = 0 ;
foreach ($local_domain_arr as $local_domain){
if($i==0){
 $pattern .= 'www.' .$local_domain.'|'.$local_domain.'|[w_]+.'.$local_domain ;
}else{
 $pattern .= '|www.' .$local_domain.'|'.$local_domain.'|[w_]+.'.$local_domain ;
}
 $i++ ;
} // bbs.it-home.org
$pattern .=')[^'^"]*['"][^>]*>(.+?)/is';
return preg_replace($pattern,'$1',$message);
}