PHP Tag Engine is a PHP library that allows you to easily add tagging to a PHP application. There is a back-end class and AJAX request handler, and a full AJAX front end interface. ADOdb is used as the database abstraction layer and PHP Tag Engine is tested on MySQL, PostgreSQL and MS SQL Server.
PHP Tag Engine was developed to add tagging to Tasks Pro™ and Tasks™. You can see it in action online in the Tasks Pro™ and Tasks™ online demos.
There is a Trac web site for this project, which includes a wiki, ticket system and SVN repository.