It seems like the output formats requested from the SPARQL endpoint are json, turtle and RDF/XML in this specific order, but the PHP code is not able to handle the turtle code.
This might be a general problem or due to the tested Virtuoso (OpenSource Version) server which might return invalid turtle.
It seems like the output formats requested from the SPARQL endpoint are json, turtle and RDF/XML in this specific order, but the PHP code is not able to handle the turtle code. This might be a general problem or due to the tested Virtuoso (OpenSource Version) server which might return invalid turtle.