TES Search is a project that grew out of a small PHP script. It has undergone successive re-writes until it is what it is today.
It is for legacy reasons written in PHP, but searching is now done using Sphinx as the full text search engine. The code uses the Zend Framework and (again for legacy reasons) the Simple HTML DOM library for parsing. A move to xpath is now being made give how resource hungry building a dom is in PHP.
The most recent version (most of the time slightly newer than the online versions) is hosted at Github under the AGPL3 licence.