• The Clean Code Talks – “Global State and Singletons”
• Why Singletons have no use in PHP
• Testing Code That Uses Singletons
• Making Singletons Safe in PHP
• The “Missing” Patterns of the PHP Manual — Singleton
• Why Singletons are Evil
• Design Patterns “You should be wary when using the singleton pattern, as by its very nature it introduces global state into your application, reducing testability.”
• “Practical Design Patterns in PHP” book “The Singleton Pattern is perhaps the most well known – and most often misused – pattern in all of PHP design pattern development. Its simplicity, combined with its seeming benefits makes it a widely-used (and overused) pattern. The Singleton is not so much a recommended pattern, as a pattern I recommend you shy away from.”
• “Singletons in PHP – Why they are bad and how you can eliminate them from your applications” presentation
• Singleton pattern – Wikipedia
• “Do I Need a Singleton” illustration

source: https://gist.github.com/GaryJones/8430080

I have some reading to do!

You can find Gary Jones on twitter, and on the web.

Edit:

After referencing this article a few times, I have accumulated more great resources.

More reading on Singletons:

• The case against Singletons in WordPress
• Singletons and their use in WordPress

More reading on Dependency Injection (DI):

• Dependency injection for WordPress plugins
• What is Dependency Injection?
• Use Dependency Injection to build your Jedi army
• What problem does dependency injection solve?
• PHP Dependency Injection