Calling functions by reference

Hi, I am trying to write a function that has parameters passed by reference, and was wondering how to get a default value to them. e.g.: function test(&$a, &$b = 0) doesn't work (PHP complains about expecting a ')' after the second parameter). Is there some other syntactically-valid way to achieve this, or do I have to provide *all* variables when I call this function? (This is a pain, as you can't statically assign values at the other end either - you have to define a temporary variable with no content). Thanks in advance, Chris.