• Link - Semantic Web Client Library

From the site:

The Semantic Web Client Library represents the complete Semantic Web as a single RDF graph. The library enables applications to query this global graph using SPARQL- and find(SPO) queries. To answer queries, the library dynamically retrieves information from the Semantic Web by dereferencing HTTP URIs and by following rdfs:seeAlso links. The library is written in Java and is based on the Jena framework.

The odcumentation for the Semantic Web Client Library is very good and has some interesting observations about the state of RDF. Unfortunately, it runs only in currently closed-source Java, but the XML examples could apply equally to PHP or any other language.