Array help please

I'm trying to debug some software that I downloaded from the web. It seems to work fine most of the time however it occasionally crashed with database errors. When I look at the code it has a lot of array references that i don't understand e.g. $Session[clientID] most references at are of the form $arrayname["item"] or $arrayname[$item] as I would expect. is the example correct code? if so what does it mean? Thanks Peter