In this recent post to the PHP::Impact blog, Federico talks about a slightly scary thing - a version of PHP implemented 100% in Java.

Quercus allows developers to incorporate Java code into PHP web applications and gives both Java and PHP developers a fast, safe, and powerful alternative to the standard PHP interpreter.

He mentions what it supports - things like Unicode and several of the popular extensions like APC, GD, MySQL, Oracle and PDF). There's also benefits he points out like the speed of the libraries (and safety) as well as interoperability with Java code and frameworks. Check out the official Quercus website for more information.