php __clone() and the "shallow clone"

Question

What is meant when the results of __clone() is a "Shallow Clone"?

Answer 1

This means that when the object is cloned, any properties that are reference variables (variables that refer to other objects, rather than a value) will remain references.

A "non-shallow" clone would set the new object's to the values of those properties, rather than leaving them as references.

Note: This means that any changes you make to those references in the cloned object will also be made to the values they reference in the "parent" object.