Idiorm is one of my favourite PHP libraries by far. Itself, plus Slim for routing on top of a bunch of classes is the nicest most maintainable way I've come across for building small web applications in PHP!

I also enjoy its companion library, Paris, an Active Record implementation on top of Idiorm.

https://paris.readthedocs.org/en/latest/

http://j4mie.github.io/idiormandparis/

RedBean is also an excellent configuration-less ORM for PHP.

http://redbeanphp.com/

what is the most robust PHP ORM out there for large projects?