PHP values inside pdo->query or/and prepared statements - pdo

Question

I'm limiting the amount of comments shown on the page by 2, $second_count counts how many posts are if more than 2 it limits and show's a Show All comments, that's what this is for.
If you look into the variable $limitPost, how do I add parameter inside it.
This is what I'm trying to accomplish but with PDO.
$limitPost = "DESC LIMIT $second_count,2"; but this can lead to SQL INJECTIONS to my understanding.
PUBLIC FUNCTION userComments($post_iD,$second_count)
{
$limitPost = "DESC LIMIT 2";
$sth = $this->db->prepare("SELECT C.com_id, C.uid_fk, C.comment, C.created, U.username, U.photo
FROM comments C, users U
WHERE U.status='1' AND C.uid_fk = U.uiD
AND C.msg_id_fk = :postiD
ORDER BY C.com_id < :second_count");
$sth->bindParam(':postiD', $post_iD, PDO::PARAM_INT);
$sth->bindParam(':second_count', $limitPost, PDO::PARAM_INT);
$sth->execute();
$data = $sth->fetchAll();
return $data;
}
UPDATE
This is what $second_count is, it it's just counting if there are 2 comments showing, it'll hide all the rest and if I press show all comments it'll expand.
<?php
$x=1;
if($x){
$comment_count = count($commentsarray);
$second_count = $comment_count-2;
if($comment_count>2){
?>
<div class="comment_ui" id="view<?php echo $post_iD;?>">
Show all <?php echo $comment_count;?> comments
</div>
<?php
$commentsarray = $Wall->userComments($post_iD, $second_count);
}
}
?>

Answer 1

alright:
If you look into the variable $limitPost, how do I add another
variable inside it.
This is what I'm trying to accomplish but with PDO.
impossible <- that is the answer to your question

Answer 2

instead of :
:second_count
try:
DESC LIMIT 2
into the prepared statement. You really do not need the parameter :second_count in your prepared statement since it is a constant string.
SQL injections would be possible if :second_count was an input by a user, since is it constant, nothing like that is possible.
Why what you are doing is not working:
DESC LIMIT 2 is not an integer parameter, so the bind should fail or give out junk.
If you were to use PDO::PARAM_STR on the bind, :second_count would be put into qoutes, so the sql statement would fail.