(PHP 5 >= 5.5.0)
SplDoublyLinkedList::add — Add/insert a new value at the specified index
   Insert the value newval at the
   specified index, shuffling the
   previous value at that index (and all subsequent values)
   up through the list.
  
indexThe index where the new value is to be inserted.
newval
       The new value for the index.
      
No value is returned.
   Throws OutOfRangeException when 
   index is out of bounds or when 
   index cannot be parsed as an integer.