On the Symfony Finland blog they have a recent entry looking at some of the other alternative PHP implementations with a brief overview of each.
In the past there have been alternative environment for running PHP in PIPP for Parrot VM and Quercus for JVM, but none of these have had real staying power. This changed with HHVM from Facebook in the early 2010's. It's completely separate from the Zend PHP implementation and is a fully featured alternative.
[...] Many the implementations are just proof of concept implementations and are far from supporting all the language features and rendering them to be useless for running Symfony or other contemporary applications. Regardless of whether these are immediately useful for projects it's interesting to follow these efforts.
The four covered in the post allow for PHP to be run in some pretty non-traditional environments:
- HippyVM letting PHP run in Python (PyPy)
- JPHP that runs PHP in Java
- Tauga VM using Rust to let PHP run compiled via LLVM
- Peachpie for running PHP on .NET
There's a brief look at each of these projects included and, while most aren't close to what HHVM has to offer, you might take a look and see if they could be interesting to try out in some of your development.