Empty reference parameter

Hi, I´m trying to give an empty reference as a parameter of a function. Or rather, make a parameter that is a reference optional. I have a function that should react differently if it is given an object or not. It works, but it gives me a warning that the second argument is not being given obviously. Sadly I cannot make the parameter optional in the function definition because mine has to be a reference. One can´t do such a thing as function( $parameter1, & objectparameter = NULL) this results into a parsing error. I cannot believe that there is no way of making an object parameter optional. So tell me please what I´m missing.