PDO count(*) does not work with LIMIT and OFFSET
Within the query itself, trying separating the COUNT(*) from the retrieval by rewriting the query like this:

SELECT COUNT(*) from (SELECT myl_news_comments WHERE news_id=:news_id LIMIT :ajax_read_how_many_comments OFFSET :items_already_loaded) A


Your code should look like this now:

        $query_list_records = "SELECT COUNT(*) from (SELECT myl_news_comments WHERE news_id=:news_id LIMIT :ajax_read_how_many_comments OFFSET :items_already_loaded) A";
        $result_list_records = $db-&gt;prepare($query_list_records);
        $result_list_records-&gt;bindValue(':news_id', $_POST['news_id'] , PDO::PARAM_INT);
        $result_list_records-&gt;bindValue(':ajax_read_how_many_comments', (int)$ajax_read_how_many_comments , PDO::PARAM_INT);
        $result_list_records-&gt;bindValue(':items_already_loaded', (int)$_POST['items_already_loaded'] , PDO::PARAM_INT);
        $result_list_records-&gt;execute();                    
        $total_records = $result_list_records-&gt;fetchColumn();

echo $total_records;


Give it a Try !!!