Replace the librdf with PHP RDF pakcage

[peter279k]

As title, the librdf is too old and this binding is not maintained actively from the website.

Consider the following PHP packages of processing RDF:

• jskos-rdf is the top of the easyrdf.
• easyrdf.

And I also need to know about the work of the RDF on this source code.

Thanks.

[peter279k]

The librdf build will be failed when detecting the PHP version is 7.0.

The related bug report is available here.

[peter279k]

If we find the current solution to fix the failed build in php-7.x, we can go ahead with the librdf extension.