PHONGO_IS_CLASS_INSTANTIATABLE is checks zend_class_entry flags and is used for two purposes:

• When encountering a __pclass Binary field in a document, only an instantiatable class can be used as the ODM class; otherwise, it is ignored and the __pclass field will show up in the deserialized PHP document.
• When specifying a class in a type map, it must be instantiatable or else an exception is raised.

I came across this when implementing PHPC-2083 as I realized the macro did not consider enums nor traits. We should add both along with regression tests for all class types presently supported (e.g. abstract, interfaces).