On the Engine Yard blog they've posted a guest article from PHP community member Chris Hartjes about some of the alternative PHP implementations that are out and available to the community.
Many PHP developers have a very vague understanding of what is meant by a “runtime”. It’s a concept that is more common in other languages languages. At it’s most basic level, it is the idea that you can have a specification for the behaviour of the language, and then you can write your own implementation of it. [...] A shocking fact: programming languages have bugs and are sometimes missing features that people desire. Alternative implementations are a great way for people to try and create a version of a language that meets their needs while hopefully remaining as backwards compatible as possible.
He talks about one of the major players in the "alternate PHP" realm right now - the HipHop Virtual Machine (or HHVM) from Facebook. The project looks to make PHP perform even better than it already does by compiling it down. Chris shares two reasons he thing they started the project - one being their own need for performance and the other being that it gives them the option of adding new features to the language they might need.