<?php
// the below code assumes you have the private/protected
// variable $this->_db set to your database adapter.

// begin the transaction
$this->_db->beginTransaction();

try {
     // mock insert into a user table
     $result = $this->_db->insert('users', array(
          'name' => 'Corey',
          'email' => 'duplicate@alreadyexists.com',
          'password' => sha1('FakeSalt' . 'FakePass'),
          'created_ts' => time(),
          'modified_ts' => NULL
     ));

     // begin process of roles insertion
     if ($result) {
          // grab the user id from the last insert
          $user_id = $this->_db->lastInsertId();
          
          // insert into a mock user roles table
          $result = $this->_db->insert('users_roles', array(
               'user_id' => $user_id,
               'role_id' => 1
          ));

          // on success, commit transaction and return the user_id
          if ($result) {
               $this->_db->commit();
               return $user_id;
          }
     }

} catch (Exception $e) {

     // THIS IS WHERE THE MAGIC HAPPENS
     // rollback
     $this->_db->rollback();

     // throw the same exception so it bubbles
     // back up to the error controller
     throw $e;
}
?>