When does a bug turn into a feature?

First, passing uninitialized variables into functions. Every variable should always be initialized to a value that is sensible for what it will be used for! It's true that PHP does not enforce this as strictly as some other dynamic languages do (Python and Common Lisp come to mind), but it's still terrible practice.

But second, and even worse, endosquid was apparently relying on undefined behavior! The documentation for number_format() does not specify what happens if you pass in a non-numeric value as its first argument, and while we could wish that said docs explicitly said "this behavior is undefined", the fact remains that it DOES say the first argument must be a float. Yes, in the past, passing in some other value resulted in "0", but the whole point of undefined behavior is that it cannot be relied upon.