From this post on the server side it seems Caucho have added PHP support to their Resin Java App server.
There are a number ways to integrate with Java, by which I mean PHP – the C version.
What’s interesting about what Caucho seem to have done is, rather than integrate with PHP-C, they’ve written their own parser for PHP scripts in Java, from which they generate Java byte code for execution. I say “seem” because I’ve only glanced at the source and found a convincing-looking PHP parser in there.
Otherwise not much information available right now except the source code itself and some minimal wiki entries.
Also, Keith raises the right questions about benchmarks.