What are the disadvantages of using a PHP database class as a singleton?

Question

What are the disadvantages of using a PHP database class as a singleton?

Answer 1

It makes it hard to run unit tests against it and also makes it impossible to have multiple database connections. As we all know, global variables has lots of drawbacks and Singletons are no exception, only that they are a more "friendly" global variable.

I found a pretty good article about it and an old SO question as well.