Quick plugin help

Im trying to edit this Hidethis plugin but Im gettng stuck. Fromt he code you see below basically anything within the <!--hidethis-->OMG HIDDEN<!--/hidethis--> is not going to show up on wordpress blogs unless they are registered.

Now I want to place some text and html where this was stripped before and I cant get it to work. Basically adding something like "text was removed because your stupid" in exchange for what was in between the hidethis tags. Any help appreciated.

only thing I can think of but didnt get it to work is something like..

$new = "content removed";
return $pre.$new.$suf;

PHP:

add_filter('the_content', 'hide_some_content');

function hide_some_content($content) {
global $current_user, $user_ID;

if (($current_user->id == 0) && ($user_ID == 0)){

    $b = strpos($content, '<!--hidethis-->');
    $e = strpos($content, '<!--/hidethis-->');

    $pre = substr($content,0,$b);
    $suf .= substr($content,$e,strlen($content));

    return $pre.$suf;

} else { return $content; }

}

PHP: