ARC2 is a PHP 5.3 library for working with RDF. It also provides a MySQL-based triplestore with SPARQL support.
Feature-wise, ARC2 is now in a stable state with no further feature additions planned. Issues are still being fixed and Pull Requests are welcome, though.
Package available on Composer.
If you're using Composer to manage dependencies, you can use
$ composer require "semsol/arc2": "dev-master"