Quercus
Quercus is a Java-based implementation of the PHP language that is included with Resin. According to a slideshow presented by Emil Ong (from Caucho), to a San Francisco Java Meetup Group in April 2008 pertaining to Resin 3.1, an essential difference in the operation of Quercus between the Resin Open Source and the Resin Professional editions is that in Resin Professional the PHP is compiled to Java bytecode whereas in the open source version PHP is executed by an interpreter.
Caucho states that Quercus is faster than standard PHP although this is only true for the JIT-compiled version in Resin Professional. Quercus ships with Resin.
Read more about this topic: Resin Server