看不懂文档这个循环为什么会输出那样的结果，有人帮忙解释一下吗？

<?php
class MyIterator implements Iterator
{
    private $var = array();

    public function __construct($array)
    {
        if (is_array($array)) {
            $this->var = $array;
        }
    }

    public function rewind() {
        echo "rewinding.";
        reset($this->var);
    }

    public function current() {
        $var = current($this->var);
        echo "current: $var.";
        return $var;
    }

    public function key() {
        $var = key($this->var);
        echo "key: $var.";
        return $var;
    }

    public function next() {
        $var = next($this->var);
        echo "next: $var.";
        return $var;
    }

    public function valid() {
        $var = $this->current() !== false;
        echo "valid: {$var}.";
        return $var;
    }
}

$values = array(1,2,3);
$it = new MyIterator($values);

foreach ($it as $a => $b) {
    print "$a: $b.";
}
?>`Copy