I have experienced "working on a copy"-problem with one object and two variables
  containing the object. Example: (Untested) 

<?
    class someobject
    {
       var $a=4;
     }

     $myobj = new someobject;

     $ref1 = $myobj;
     $ref2 = $myobj;

     $ref2->a = 5;
 ?>

Now ref1 is a someobject where a=4 and ref2 is a someobject where a = 5. I would expect that ref1 and ref2 points to the same object and thus the assignment ref1->a=5 will cause the object referenced by ref1 and ref2 to be updated since it is the same object. <br><br>
This is very annoying and the whole point of having objects and classes seems to disappear, except for their "functionality-wrapping" features.