Prefix and mask any filtered external link with Referrer spoofing or hiding service. The original idea came from WordPress.com Link Visibility option.
Add the following lines to your WordPress Theme’s template functions.php file (For example, functions.php file for a Theme named “default” would probably reside in the directory wp-content/themes/default/functions.php) or create it as a Plugin by filling the additional Standard Plugin Information:
<?php
function referrerspoofing($link) {
$service = 'http://hiderefer.com/?';
$exclude = '('.get_option('home').'|'.$service.')';
return preg_replace('#<a(s+.*?href=S(?!'.$exclude.'))#i', '<a$1'.$service, $link);
}
add_filter('the_content', 'referrerspoofing');
add_filter('comment_text', 'referrerspoofing');
add_filter('get_comment_author_link', 'referrerspoofing');
add_filter('wp_list_bookmarks', 'referrerspoofing');
?>
From the snippet above, any filtered external link within the_content(), comment_text(), get_comment_author_link(), wp_list_bookmarks() will need to pass through hiderefer.com. For example, http://example.com will transform to http://hiderefer.com/?http://example.com.