If you've been following along with some of the articles about the compiler Facebook is developing for PHP, you might want to check out these comments from ReadWriteWeb.com (by Mike Melanson) with more from his point of view.
According to our sources, Facebook has been working on a PHP compiler that will increase speed by around 80% and offer a just-in-time (JIT) compilation engine that will offer a number of advantages. The project is very similar to Google's Unladen Swallow project, which rebuilt the Python compiler, boosting the speed fivefold and opening the door for multi-language integration.
Included are also some comments from Richard Crowley about what Facebook could be releasing, a compiler that would still allow PHP to remain interpreted but provide the speed of being compiled. Some of the comments on the post wonder how something like this compiler compares to an opcode cache and doubts on whether it will end up being open sourced at all.
You can also get another opinion on what will be released in this article from the All Facebook blog. They reinforce that the announcement will come today and how much potential something like this could have to change what PHP could become.