This information is not easily available from the engine and tracking it would be quite expensive. If you need it, you can do it using something like this: (untested)

$tracked_objects = array(); // Global

class TrackedClass {
    public function __construct() {
       $GLOBAL['tracked_objects'][spl_object_hash($this)] = array(
           'class' => get_class($this),
           'stack' => debug_backtrace()
       );
    }
    public function __destruct() {
       unset($GLOBAL['tracked_objects'][spl_object_hash($this)]);
    }
}

Any class you want to track can than be extended from such a class (or use 5.4 and traits) andyou can inspect $tracked_objects.