- Google's Clean Code Talks
- http://blog.gordon-oheim.biz/2011-01-17-Why-Singletons-have-no-use-in-PHP/
- http://sebastian-bergmann.de/archives/882-Testing-Code-That-Uses-Singletons.html
- http://eamann.com/tech/making-singletons-safe-in-php/
- http://hakre.wordpress.com/2012/06/10/the-missing-patterns-of-the-php-manual/#p2
- http://blogs.msdn.com/b/scottdensmore/archive/2004/05/25/140827.aspx
- http://www.phptherightway.com/pages/Design-Patterns.html "You should be wary when using the singleton pattern, as by its very natureit introduces global state into your application, reducing testability."
- http://www.practicaldesignpatternsinphp.com/ "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."
- http://www.slideshare.net/go_oh/singletons-in-php-why-they-are-bad-and-how-you-can-eliminate-them-from-your-applications
- http://en.wikipedia.org/wiki/Singleton_pattern
- http://i.stack.imgur.com/Dzdio.png
Once you've read all those, read everything abou Dependency Injection, how that helps with injecting mocks during unit testing, and therefore why Singletons are bad.
I know all the drama they say that Singleton is anti-patron and the whole Show, But this is a technique that is used, can not be considered bad when it helps your Plugin or Software to be faster and consume less ram