I ran into a problem when using the statement-centric find(…) query using the OntModel with the RDF Vocabulary. The specified RDF resources were treated like NULL in the find function. In order to fix this, create a ResProperty instead of a ResResource, and all will be well. This can be done like so:
$rest = $this->ontModel->find($statement->getObject(), new ResProperty(RDF_NAMESPACE_URI . 'rest'), NULL);
References
My open source contribution, a bug report 
: ResResource as parameter for ResModel::find does not work.
