I found an interesting project, quercus that compiles PHP to Java Bytecode allowing it to run ontop of the Java Resin Application Server. It has a couple neat features, one being PHP6 compatible unicode support, the other being easy integration between Java and PHP code.
He gave it a shot and notes that the performance seems decent and allows you to compensate for some of the differences in the PHP API it supports and what you're using via a Java class. According to their site:
Quercus is Caucho Technology's 100% Java implementation of PHP 5 released under the Open Source GPL license. Quercus comes with many PHP modules and extensions like PDF, PDO, MySQL, and JSON. Quercus allows for tight integration of Java services with PHP scripts, so using PHP with JMS or Grails is a quick and painless endeavor.
It includes several extensions too, including the JSON library, APC, MySQL and Oracle support, and PDF functionality. Check out their Getting Started guide, documentation, or their wiki for more information.