ARC is a lightweight, SPARQL-enabled RDF system for mainstream Web projects. It is written in PHP and has been optimized for shared (or performance- or privilege-limited) Web environments. ARC facilitates the integration of RDF and SPARQL into PHP/MySQL frameworks.
ARC is a very well documented project, which I think is important for its success. Their documentation is clear, concise, and provides plenty of inline sample code to examine as you read and learn.