PDO - having trouble inserting data using a class - php

Question

I'm having a little trouble making something work with a PDO. I will explain as I provide the code.I'm not expecting an answer, but I'm hoping that somebody could provide an explication on what I'm doing wrong and how a correct solution would work like.
I'm working on a CMS and trying to implement a MVC pattern, hence the use of PDOs.
Here is the section of my index.php file where we have the PDO declaration:
<?php
include_once 'models/USER_mapper.php';
$USER_mapper = new USER_mapper( 'localhost', 'root', 'root', 'online_magazine' );
$user = new User();
$user->setIDUser("JohnnyBoy");
$USER_mapper->registerUser($user);
?>
In the USER_mapper I have the connect function which is called on creation of a new USER_mapper and I also have the registerUser() function which takes a User object as a parameter and it should insert into the DB the info from the User object that was used.
Below is the registerUser() function:
public function registerUser($userOBJ) {
try {
$q = $this->DBH->prepare ( "INSERT INTO online_magazine.USER_TABLE (id_user, permission) VALUES (:id_user, :permission)" );
$q->execute ( ( array ) $userOBJ );
$this->DBH->commit ();
echo "it works";
} catch ( Exception $e ) {
echo "it doesn't work";
echo $e->getMessage ();
}
}
The table into which I am inserting has two columns: ID_USER and PERMISSION. Permission's has a default value, which is why I only provide the ID_USER with the setter function and leave the permission blank.
What I'm trying to achieve is to insert data into a table using a User object that has the right values and passing the User object to USER_mapper which will recuperate the info and put it in the respective table.
I hope I have been clear enough to highlight my problem. Let me know if I should add more detail and where.
EDIT1: I forgot to mention that when I use the method above I get a php error that looks like this:
[22-Nov-2013 03:18:08 Europe/Berlin] PHP Fatal error: Uncaught exception 'PDOException' with message 'There is no active transaction' in directory
EDIT2: I found out how to do it using bindParam() as follows:
$id_user = $userObject->getIDUser ();
$this->DBH->beginTransaction ();
$stmt = $this->DBH->prepare ( "INSERT INTO online_magazine.USER_TABLE (id_user) VALUES (:id_user)" );
$stmt->bindParam ( ':id_user', $id_user, PDO::PARAM_STR );
$stmt->execute ();
$this->DBH->commit ();
Thank you all!!