Imagine the possibilities when a PHP developer can continue using the language's templating capabilities while leveraging Hibernate's object-relational features. Thanks to an open source project named Quercus, PHP and Java developers alike can mix and match the respective capabilities of both languages. [...] This approach (part of the movement that software architect Neal Ford famously defined as "polyglot programming") opens up a whole new world of opportunities for developers seeking to squeeze every last ounce of performance and capability from their application platforms.

They help you get it installed and show you how to execute a PHP script from inside. There's also a bit about integrating Java libraries into your code and the integration of a type of tool PHP doesn't come native with, something like iTextPDF.