다음 Iterator 클래스의 구현에서 메서드의 역할은?

<?php
class NumberIterator implements Iterator {
    private $numbers = [10, 20, 30];
    private $position = 0;
    
    public function current() {
        return $this->numbers[$this->position];
    }
    
    public function key() {
        return $this->position;
    }
    
    public function next() {
        $this->position++;
    }
    
    public function rewind() {
        $this->position = 0;
    }
    
    public function valid() {
        return $this->position < count($this->numbers);
    }
}

$iterator = new NumberIterator();
foreach($iterator as $key => $value) {
    echo "$key: $value ";
}
?>