Quercus is Caucho Technology’s fast, open-source, 100% Java implementation of the PHP language. Quercus is a feature of Caucho Technology’s Resin Application Server and is built into Resin – there is no additional download/install. Developers using Resin can launch PHP projects without having to install the standard PHP interpreter (http://www.php.net) as Quercus takes on the role of the PHP engine.
What is Quercus

Quercus implements PHP 5 and is internationalization/localization (i18n/l10n) aware. Quercus natively supports Unicode and the new Unicode syntax of the up-and-coming PHP 6. Quercus implements a growing list of PHP extensions (i.e. APC, iconv, GD, gettext, JSON, MySQL, Oracle, PDF, Postgres, etc.). Many popular PHP applications will run as well as, if not better, than the standard PHP interpreter straight out of the box. The growing list of PHP software certified running on Quercus includes DokuWiki, Drupal, Gallery2, Joomla, Mambo, Mantis, MediaWiki, Phorum, phpBB, phpMyAdmin, PHP-Nuke, WordPress, and XOOPS.