The original implementation of PHP has a PHP/Java Bridge separately available from SourceForge. This uses a dual process model where the Java libraries are run in a separate process from the PHP runtime. Interactions with Java objects are marshalled across to the Java process using an XML protocol.
Two other implementations, Quercus and Project Zero, implement PHP on top of the Java Virtual Machine and so can run the Java libraries in the same process as the PHP runtime. This reduces the overhead of interacting with Java libraries when compared with the dual process model of the original implementation.