Over the second half of 2008, I spent quite some time working with John Lim, ADOdb's project lead; first, adding one-to-many relationships to ADOdb; then, merging this code in his active records code.
The resulting library was quite powerful.
Due to its adhering to ActiveRecord's "convention over configuration" philosophy, it was then turned into two different entities: Active Record, which was more in line with PHP's verbose interfaces, and Active Record eXtended, which relies purely on convention.
When you download ADOdb for PHP, you automatically get these classes.
I have not, so far, written a lot about ADOdb's ActiveRecord on this blog. John did a better job, for instance here.
I posted some comments on this support thread and both John and I are available it you need any help with Active Records.