在foreach循环期间如何调用迭代器接口中的方法,以及它们的返回值会发生什么变化?[闭门]

这些方法是如何和何时使用的 current() ; key() next() 和 valid() 打电话?它们的返回值是否用于任何用途?

PHP Manual :

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

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

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

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

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

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

    public function valid()
    {
        $key = key($this->var);
        $var = ($key !== NULL && $key !== FALSE);
        echo "valid: $var\n";
        return $var;
    }

}

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

foreach ($it as $a => $b) {
    print "$a: $b\n";
}

上述程序的输出为:

rewinding
valid: 1
current: 1
key: 0
0: 1
next: 2
valid: 1
current: 2
key: 1
1: 2
next: 3
valid: 1
current: 3
key: 2
2: 3
next:
valid: 

我正在使用 PHP 7.3.1