I just finished a basic PHP port of ActiveResource which I put online here:
It's a basic class, not 100% feature-complete but it does cover the basic CRUD functions and executing custom methods (e.g., /person/5/custom_method.xml?key=value), and it uses a fairly similar coding style to the Rails code, for example:
$obj = $objects->find ($key);
$obj->title = 'New title';
And you can chain things like this too:
$obj->find ($key)->set ('title', 'New title')->save ();
Anyway, hope it helps save people some time integrating Rails-based services with their PHP code. Also feel free to make comments, suggestions, improvements, etc.