The PHP to Java conversion process creates code linked to a separate library (provided as a .jar) that provides various helper functions and classes used to make the conversion process work. It has a JNI connection to a library that connects to the real PHP codebase and forwards basic PHP standard library + extension function calls. This also means that the library will only work on those platforms for which a JNI library file is provided.
For the first release, we will be providing library files for macOS and for Ubuntu 16.04 (64-bit). This narrow choice of platforms will make it easier to support the public release.