Doctrine is (as they describe it on the website):

Doctrine is a PHP ORM (object relational mapper) for PHP 5.2.3+ that sits on top of a powerful PHP DBAL (database abstraction layer). One of its key features is the ability to optionally write database queries in an OO (object oriented) SQL-dialect called DQL inspired by Hibernates HQL. This provides developers with a powerful alternative to SQL that maintains a maximum of flexibility without requiring needless code duplication.

The tutorial walks you step-by-step through the process of integrating Doctrine with CodeIgniter and then dives even deeper into topics such as model relationships, hooks, profiling, DQL, templates, and data hydrators. Overall, it’s a very thorough tutorial.

• Day 1: Install and Setup
• Day 2: The Basics
• Day 3: User Signup Form
• Day 4: User Login
• Day 5: Database CRUD
• Day 6: Models with Relationships
• Day 7: Fixtures & Forum List
• Day 8: Hooks, Profiling & DQL
• Day 9: Templates & Data Hydrators
• Day 10: …coming soon

FYI – In the end I decided against using Doctrine in my project, as it added a bit too much weight and required that I mess with yet another layer. But you stand to lose nothing going through these tutorials, you will learn a lot even if you decide against Doctrine in the end.