Ok,looks like I was using the wrong way to parse.
$aString = '
<span vocab="http://schema.org/" typeof="Document">
<a property="url" href="http://www.w3.org/TR/rdfa-primer/">
<span property="title">RDFa 1.1 Primer</span></a>.
</span>';
// Extracting RDFa from HTML
$config = array('auto_extract' => 0);
$parser = ARC2::getSemHTMLParser();
$base = 'http://example.com';
$parser->parse($base, $aString);
$parser->extractRDF('rdfa');
$triples = $parser->getTriples();
$rdfxml = $parser->toRDFXML($triples);
print_r($rdfxml);