How to implement PHP foreach to traverse multi-dimensional arrays

Introduction
Normally our foreach can print out each key => value in the one-dimensional array in order, but if it is a multi-dimensional array, it needs to loop in a nested loop, or implement it recursively, but these methods are not flexible enough. Because it is impossible to have an endless nested loop without knowing how many dimensions the array has. If we use recursion, we can solve it, but if we just want to use foreach to loop out all the loops, how can we achieve it?

Implementation method one
Use PHP’s own iterator class RecursiveIteratorIterator

$test_arr = array(1,2,3,array(4,'aa'=>5,6,array(7,'bb'=>8),9,10),11,12);
 $arrayiter = new RecursiveArrayIterator($test_arr);
 $iteriter = new RecursiveIteratorIterator($arrayiter);
 //直接打印即可按照横向顺序打印出来
 foreach ($iteriter as $key => $val){
   echo $key.'=>'.$val;
 }
 //结果
 /*
   0=>1
   1=>2
   2=>3
   0=>4
   aa=>5
   2=>6
   0=>7
   bb=>8
   4=>9
   5=>10
   4=>11
   5=>12
*/

Implementation method two
Implement an iterator class similar to RecursiveIteratorIterator by yourself to realize the horizontal printing function of multi-dimensional arrays

class foreachPrintfArr implements Iterator {
    //当前数组作用域
    private $_items;
    private $_old_items;
    //保存每次执行数组环境栈
    private $_stack = array();
 
    public function __construct($data=array()){
      $this->_items = $data;
    }
 
    private function _isset(){
      $val = current($this->_items);
 
      if (empty($this->_stack) && !$val) {
        return false;
      } else {
        return true;
      } 
    }
 
    public function current() {
      $this->_old_items = null;
      $val = current($this->_items);
 
      //如果是数组则保存当前执行环境，然后切换到新的数组执行环境
      if (is_array($val)){
        array_push($this->_stack,$this->_items);
        $this->_items = $val;
        return $this->current();
      }
 
      //判断当前执行完成后是否需要切回上次执行环境
      //(1) 如果存在跳出继续执行
      //(2) 如果不存在且环境栈为空，则表示当前执行到最后一个元素
      //(3) 如果当前数组环境下一个元素不存在,则保存一下当前执行数组环境 $this->_old_items = $this->_items;
      //然后切换上次执行环境 $this->_items = array_pop($this->_stack) 继续循环, 直到当前数组环境下一个
      //元素不为空为止
      while (1) {
        if (next($this->_items)) { 
          prev($this->_items); break;
        } elseif (empty($this->_stack)) {
          end($this->_items); break;
        } else {
          end($this->_items);
          if (!$this->_old_items)
            $this->_old_items = $this->_items;
          $this->_items = array_pop($this->_stack);
        }
      }
 
      return $val;
    }
 
    public function next() {
      next($this->_items); 
    }
 
    public function key() {
      // 由于 key() 函数执行在 current() 函数之后
      // 所以在 current() 函数切换执行环境 , 会导致切换之前的执行环境最后一个 key
      // 变成切换之后的key , 所以 $this->_old_items 保存一下切换之前的执行环境
      // 防止key打印出错
      return $this->_old_items ? key($this->_old_items) : key($this->_items);
    }
 
    public function rewind() {
      reset($this->_items);
    }
 
    public function valid() {                                      
      return $this->_isset();
    }
  }

Internal execution method

1. Foreach loops our customized foreachPrintfArr class, which will automatically call the five internal methods valid(), rewind(), key(), next(), and current(). We only need Just implement these methods.

2. Calling sequence:
The 1st time=> rewind -> valid -> current -> key
The 2nd time~n times=> next -> valid -> current -> key

$test_arr = array(1,2,3,array(4,'aa'=>5,6,array(7,'bb'=>8),9,10),11,12);
$iteriter = new foreachPrintfArr($test_arr);
foreach ($iteriter as $key => $val){
  echo $key.'=>'.$val;
}
//结果：
/*
0=>1
1=>2
2=>3
0=>4
aa=>5
2=>6
0=>7
bb=>8
4=>9
5=>10
4=>11
5=>12
*/