On the SitePoint PHP Blog today Troels Knak-Nielsen asks th3e question "what's so bad about the singleton?" For all of its advantages, is there a darker side of the design pattern when it pertains to global variables.

As I have often taken this stance myself, I found it reasonable that I should be able to argue for it, so I'll try to give an explanation. This is also in part a follow-up on my post from last week, in which I present a way to avoid global symbols, without spending much time on why.

He talks about what they are and how they're commonly used - sometimes with some unpleasant side effects because of their use of globals.