optional object arguments to a function

I've started working on a class using DOMDocument to assemble MathML in 
php. The class, assuming I actually succeed, will eventually be used for 
parsing LaTeX math equations to MathML without the need to have TeX 
installed. I probably won't be able to support all the possibilities for 
equations that LaTeX does w/o a TeX install (and definitely not user 
defined macros) but I suspect I can (hopefully) cover most of the common 
stuff.

One thing I don't know how to do, though, is write a function where 
arguments are optional object.

IE for a function to generate an integral, the limits are optional but 
if specified must be an object (since they may be an equation 
themselves). I want the default to be some kind of a null object so I 
know to do nothing with it if it is null.

With string/integer you just do

function foo($a='',$b='',$c=false) {
   }

How do I specify a default null object, or otherwise make the argument 
argument optional?