The RedBeanPHP4 framework is probably the easiest to learn open source ORM framework in PHP. In the professional PHP course I deliver in HIT I chose to teach how to use this framework instead of Propel. It is a simple framework both for teaching and using.
I have recently completed to prepare the training material for teaching how to use this framework. You can find the new RedBeanPHP Basics course available for free personal and academic usage at abelski.lifemichael.com.