Over on Reddit.com there's an interesting post (yes, despite some of the comments made on it) about variable variables that turns into a detailed look at PHP variable handling as based around "$".

OK, here's the thing: [variable variables are] only the entrance of the rabbit hole. [...] The first thing to understand is what $ is. $ is actually a shorthand for ${} and means "return the value of the variable whose name is contained in this".

The response goes on to talk about variable names as strings, a few string handling tricks that can be used when referencing them and how this works with objects too. Things get a bit more complicated when they start adding in more levels of "variable" and evaluations based on random results. Not overly useful information for the casual developer, but interesting to know none the less.