PDO query werkt niet

<?php
$servername = "server";
                                  $username = "gebruikersnaam";
                                  $password = "wachtwoord";

                                  try {
                                      $dbh = new PDO("mysql:host=$servername;dbname=blog_categories", $username, $password);
                                      // set the PDO error mode to exception
                                      $dbh->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
                                      echo "Connected successfully";
                                      }
                                  catch(PDOException $e)
                                      {
                                      echo "Connection failed: " . $e->getMessage();
                                      }

                                      $sql=$dbh->query("SELECT blog_categories . * , COUNT( blog_posts.id ) AS count FROM blog_categories LEFT JOIN blog_posts ON blog_posts.id = blog_categories.ID GROUP BY blog_categories.ID ORDER BY count DESC LIMIT 0 , 30");
                                      while($row=$sql->fetch(PDO::FETCH_OBJ)) {
                                        echo $row->title.':'.$count;
                                      }
?>