COW in PHP

Copy-on-write (sometimes referred to as "COW") is an optimization strategy used in computer programming. The fundamental idea is that if multiple callers ask for resources which are initially indistinguishable, they can all be given pointers to the same resource. This function can be maintained until a caller tries to modify its "copy" of the resource, at which point a true private copy is created to prevent the changes becoming visible to everyone else. All of this happens transparently to the callers. The primary advantage is that if a caller never makes any modifications, no private copy need ever be created.

Recently, I was dealing with large data sets in PHP and I was wondering how PHP's garbage collection works. I discovered that PHP actually uses copy-on write (COW)!

Consider the following script, which demonstrates how PHP's GC works.
Note that before PHP 5.3.0, circular memory references result in memory leaks.