Your design looks broken. Why should unset a random pointer to the object destroy it? If you really need to unset them - which I'd most likely consider broken design - add a method to invalidate it this ...

If you really want to do this the needed function is available - spl_object_hash().

But mind that we don't have any true unique identifier - once an object is destroyed it's ID might be re-used by others.

2 johannes
So you think, that I need to force call $object->invalidate() / garbage collection 
each time I want no memory leaks? Well, I don't need a "unique id", just "lifetime 
unique id".
spl_object_hash() don't have a reverse function spl_get_object_by_hash()

I can describe it better:

I know that when I'm calling $object->property->method(), $object IS STILL 
ALIVE, but I can't access $object in this method() because if I'll hold 
reference to $object in this $property, I'll be required to call GC or destroy 
$object and all properties manually:

foreach ($iterator as $object){
  // do something
  $object->destroy(); // ... foreach ($this->properties as $property) $property-
>destroy();
}
...
function doSomething(){
   $object = new object();
   $object2 = new object();
   $object3 = new object();
   $object4 = new object();
   // do something
   destroy($object, $object2, $object3, $object4);
}