Even though Web Semantic, RDF, and Sparql has been promoted by W3C as part of Web 3.0 and Semantic Web ideas, it seems that it doesn't attract many people to develop library to support programmers. This is the same problem that became my reason to do my thesis for my master degree. At that time, my problem is the lack of working Annotea Client.
One of my student is now working on his thesis as his final year project and he was unable to find RDF/SPARQL API for PHP, but with only one search, i can find RAP - RDF API for PHP, which i think is another outdated (but still working) PHP library to support RDF and SPARQL.
I wish there are more libraries to support the continuation development of Semantic Web and Web 3.0