In this new post to his blog Gonzalo Ayuso looks at building an ORM (don't worry, he recommends something like Doctrine first) as an exercise to understand how they're constructed and how one could fit his goals.

What’s the motivation for me to build this ORM? The answer is a bit ambiguous. I like SQL. It allows us to speak with the database in a very easy way. [...] So the idea I figured out was to create a set of classes based on my tables, in a similar way than traditional ORMs to help me to autocomplete the fields and table names.

He creates a simple example with a "test" table with three columns with a mapped class (in the "Orm" namespace) that will allow IDEs to follow down the path to fetch the data from the "id", "field1" and "field3" columns. The complete code listing for his example is at the end of the post - PHP 5.3 friendly, of course. Some trigger and scaffolding examples are also included.