In order to effectively implement our solution for creating sortable object collections, we need to determine what common properties the collection classes will have as well as the requirements for the objects that will be contained in a collection. For now, let us define two interfaces - ICollection for the collection classes and ISortable for the objects that need to be sorted. You will see the meaning of the ISortable name shortly.

interface ICollection
{
     public function Get($i);
     public function Append($object);
     public function Sort();
}

For now we are only going to support the three methods defined in the above interface. Get() will return an element by index, in lieu of an indexer. Append() will add a new element to the collection. Sort() will, of course, sort the elements in the collection.

interface ISortable
{
     public function GetSortKey();
}

This interface will be implemented by the objects inside the collection that need to be sorted. The GetSortKey() method will return the "key value" of the object, similarly to the way ToString() works in .NET or Java.

Now we need to define a basic abstract base class for our collection classes.

abstract class Collection implements ICollection
{
     protected $data;  

     public function __construct()
     {
          $this->data = new ArrayObject();
     }    

     public function Get($i)
     {
          return $this->data[$i];
     }    

     public function Append($object)
     {
          $this->data->Append($object);
     }    

     public function Sort()
     {
     }
}

Here we have a basic implementation of the Collection class. It provides a default Get() and Append() method as well as a default constructor. Notice that the default implementation for the protected member $data is an ArrayObject. I chose this simply because I would prefer to be consistent and work with an object than the primitive type since both expose indexers. The only difference between using the ArrayObject and using an actual array is that we cannot directly access the inner array, which won't be a problem.