Clarification on creating an interface vs using a predefined interface

[ifubad]

The following interface, because it starts with "interface",  is that considered as a programmer defined interface, and not PHP's predefined interface? Book I am reading is getting me confused. Thank in advance

 

<?php
interface Iterator{
    public function current();
    public function key();
    public function next();
    public function rewind();
    public function valid();
}
?>