Derick Rethans is republishing an article series he wrote (originally for php|architect) about the garbage collection that is included with the PHP 5.3 releases. He kicks off the series with this first post introducing internal variable handling.
Before we start with the intricate details of PHP's new GC engine I will explain why it is actually needed. This, combined with an introduction how PHP deals with variables in general is explained in this first part of the column. The second part will cover the solution and some notes on the GC mechanism itself, and the third part covers some implications of the GC mechanism, as well as some benchmarks. But now first on to the introduction.
He introduces the concept of a "zval" - the container PHP uses internally to handle variables (along with its "is_ref" and "refcount" to tell the interpreter if it's a reference or not). He also shows how these relate to the variables you set in your applications as well as a mention of the xdebug_debug_zval function of XDebug to show how it's handled behind the scenes. He also shows how references are handled with accompanying images to show the flow. If you'd like more information on variable handling, Derick points to this article for more detail.