MySQL Last Insert ID for PDO Users

<?php
  $host = 'host';
  $user = 'user';  
  $pass = 'pass';
  $db = 'db';
  try {
    $conn = new PDO("mysql:host=$host;dbname=$db", $user, $pass);
    // set the PDO error mode to exception
    $conn->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
    $sql = "INSERT INTO users (username, password, email) VALUES ('Johny', 'Dawkins', '[email protected]')";	
    // use exec() because no results are returned
    $conn->exec($sql);
    $latest_id = $conn->lastInsertId();
    echo "Insert successful. Latest ID is: " . $latest_id;
  } catch(PDOException $e) { 
    echo $sql . "<br>" . $e->getMessage(); 
  }
  $conn = null; 
?>