Yes SPL Autoloader will load classes and interfaces. Once the autoloader is triggered you can use any reference to an auto-loadable asset to trigger the load
My_Special_Class::SOME_CONSTANT
will trigger a load as well as calling or referencing any visible static method or property.
In fact exploiting this is one way to trigger the auto_loading of namespaced functions. Define a class file like this.
namespace My\Namespace;
abstract class Functions{
const LOADED = true;
}
function func1(){}
function func2(){}
function func3(){}
And in your code when you need the functions defined in My\Namespace simply
if (\My\Namespace\Functions::LOADED){
func1();
func3();
}
The reference to the abstract class triggers the autoloader to include the file that defines the functions.