Jason Gilmore on Developer.com has posted a nice introduction to Quercus, which is Caucho Technology's 100% Java implementation of PHP 5 released under the Open Source GPL license:
It reminds me of JRuby, which I use everyday. The great benefit of technologies like these is that they enable the developer to avoid the verbosity of the Java language itself in favour of a dynamic language like PHP or Ruby, but still leverage the highly optimised Java interpreter (JRE).
Updated 2021 : note that the above post was originally published in 2010, but is left here for archival purposes. While the Quercus site is still running as of 2021, I can see that there has not been a software release since 2014. The Developer.com article is gone, so I have updated the link to point to an archived copy from the Wayback Machine.