Custom SOAP Types of Custom SOAP types?

I have a wierd problem that I've been banging my head against for some time. I'm trying to simulate inheritance with SOAP (if SOAP does this I haven't found any information about it). So I have an object that has 4 fields only 1 of which will be populated at any time. These fields represent derived objects. When I populate one of these fields and then build my SOAP_Value I get this: - <ns4:GetPlayPlanResponse> - <PlayPlan xsi:type="SOAP-ENC:Array" SOAP-ENC:arrayType="ns4:IntervalPlayPlan[1]" SOAP-ENC:offset="[0]"> - <IntervalPlayPlan xsi:type="ns4:IntervalPlayPlan"> ---DataRemoved--- </IntervalPlayPlan> </PlayPlan> </ns4:GetPlayPlanResponse> What I think I should be getting back is this: - <ns4:GetPlayPlanResponse> - <PlayPlan xsi:type="ns4:PlayPlan"> - <IntervalPlayPlan xsi:type="ns4:IntervalPlayPlan"> ---DataRemoved--- </IntervalPlayPlan> </PlayPlan> </ns4:GetPlayPlanResponse> Has anyone run into anything like this? Or does anyone know if I can build Inheritance into my SOAP Types? I've been able to force PEAR::SOAP to generate the second response if I add a simple type to my PlayPlan object, or if I set more than 1 of my fields to a value. But when I only have one field populated and the others are null I get that encoding section. Any help is appreciated, Rob