Description:
------------
When I return a graph of objects that has circular references (a tree of categories, for example), the soap server gets stuck in an infinite loop/recursion and hangs.

Reproduce code:
---------------
http://dev.plauditdesign.com/~hjiverson/drop/php-soap-multiref-bug.txt

Expected result:
----------------
The server should use mutliRefs and create references to each object. Each object instance would be serialized one time and given a unique ID, which is then pointed to each time an reference to that object is encountered. 

Something like this:

<getThingWithParentResponse>
  <result href="#object1"/>
</getThingWithParentResponse>

...

<multiRef xsi:type="ThingWithParent" id="object1">
   ...
   <parent xsi:nil="1"/>
   <children>
     <child href="#object2"/>
   </children>
</multiRef>

<multiRef xsi:type="ThingWithParent" id="object2">
   ...
   <parent href="#object1"/>
   <children xsi:nil="1"/>
</multiRef>

Actual result:
--------------
php hangs