Joe Watkins, core contributor and pthreads author, is working on a new threading API for PHP called parallel. It might be a while before it is released.
Recently, I set to work on a new threading API, named Parallel, it is not an exact clone of any existing threading API, it is an API focused on being simple and hiding the complexity inherent in utilising parallelism in your application, it is also focused on being forward compatible with the JIT, for that day when we can actually execute machine code in userland and in parallel.
EDIT: meanwhile a stable version has been released. You can find it in this repo on GitHub.