In this recent post over on DZone.com Giorgio Sironi takes a look at the "clone" feature of PHP - what it is, how it can be used and things to watch out for in its use.
Cloning is an operation consisting in the duplication of a data structure, usually to avoid the aliasing problem of having different code modify the same instance in inconsistent ways. In PHP, cloning can be accomplished in multiple ways - and in some cases it can be avoided altogether.
He talks some about how objects are passed around internally during the PHP execution and how you can tell if a function works with data by reference (from the manual). He then looks at the "clone" keyword and what kinds of things are duplicated from an object when it is used. He briefly touches on the "__clone" magic method for solving the "shallow clone" problem and how, possibly, serializing the object might be a better alternative for reproducing the entire object.