You will likely need to get your head around SPARQL in order to break up the RDF and iterate through the file.
This search RDF to SQL PHP turns up some solutions, including ARC2 which is designed for that type of work. Another search: PHP ARC2
Setting it up might be a bit of a pain.
IIRC there were online tools around to do this a few years ago.
If the RDF is coming from a datastore with a SPARQL endpoint (which they usually do) then you might be able to grab the data over an API, and do it in chunks too.