Check out mink and it's support for zombie as a back-end.
It looks quite good, and appears to be well supported.
Edit:
Unfortunately, php has no native javascript interpreter; so if you are looking for a pure php implementation including javascript support, currently, you will not find it. For headless with javascript support, you will have to rely on something like node.js or phantomjs on the back-end.
I'm sure you've seen them already, but for other options that are either headed or lack javascript support, Mink's other backends are worth perusing.