Java Specification Request 223 (JSR223), put together by representatives from Sun, Zend, Macromedia, and Oracle, details a plan to provide a solid API in Java that allows PHP programmers to dig deeper into Java when they work in tandem on a web server. If JSR223, once refined, is accepted into the Java system, it will mean that PHP developers have an easy way to connect their front-ends with existing back-end Java business logic that is currently not possible.
The advantage of this is clear - both Java and PHP actually work to complement each other remarkably well, given that both are cross-platform, both have a slant towards the web, and both are available free. Many businesses use Java for their business systems because that decision makes sense - Java excels in this role. However, these same businesses have been stuck with Java for their front-end tasks, such as serving up dynamic web pages - tasks that PHP does much better.
If Java and PHP were to be made almost interchangeable on the server level, companies could carry on using their complex Java structures behind the scenes, but layer flexible PHP code on top of it all, with that PHP having full ability to interact with the Java objects.
Sun are actively pushing this JSR, although one has to question their motives - whether or not they see this as just another way to beat down Microsoft's ASP.NET initiative is hard to say.