Calendar and events are just regular content objects and you can use eZ Publish PHP API to access them in your code.
Take a look at here http://pubsvn.ez.no/doxygen/4.4/html/ , specifically the documentation for eZContentObject and eZContentObjectTreeNode PHP classes.
For example, you can use eZContentObjectTreeNode::subTreeByNodeID(false, 123) static function to fetch all nodes below node with node_id = 123
