A graph consists of edges and nodes. In Pho, the core components edges and nodes are organized as subclasses of Entity for the common themes they share (such as an identifier, label etc). Graph is positioned completely different, and SubGraph stands uniquely as a subclass of Node that also shows Graph traits at the same time.
Graph illustration created with yEd.
Pho is written in PHP 7. All files in pho-lib-graph (with the single exception of the EdgeList.php) are in strict mode enabled by default.