$sql="SELECT * FROM `customer_information_table` WHERE `email`='{email}'";
                $result = $db->query($sql);
                      if($db->error)
                        {
                            exit("SQL error.
                                 <a href = './register.php'>return</a>");
                        }

                      if($result->num_rows!==0)
                      {
                        exit("email address already exists.
                             <a href = './register.php'>return</a>");
                      }

I picked up a few lines that might not have the desired return you want, try the following...

$email = here@there.com; //Sample and not checked for validation...
$sql="SELECT * FROM `customer_information_table` WHERE `email`='".$email."'";
                $result = $db->query($sql);
                      if($db->error)
                        {
                            exit("SQL error.
                                 <a href = './register.php'>return</a>");
                        }
                      if(count($result) > 0)
                      {
                        exit("email address already exists.
                             <a href = './register.php'>return</a>");
                      }