Three PHP multidimensional array to one dimensional array method instances

Many times we need to convert a multidimensional array to a 1-dimensional array, because we only need a 1-dimensional array, and 1-dimensional arrays are easier to use. How do we convert a multidimensional array to a 1-dimensional array in PHP? Let's take a look at three examples of converting multidimensional arrays to one-dimensional arrays:
1. Use foreach

<?php
   function arr_foreach ($arr) 
   {
      static $tmp=array();  
      if (!is_array ($arr)) 
      {
         return false;
      }
      foreach ($arr as $val ) 
      {
         if (is_array ($val)) 
         {
            arr_foreach ($val);
         } 
         else 
         {
            $tmp[]=$val;
         }
      }
      return $tmp;
   }
   $a = array(1,2=>array(3,4=>array(5,6)),7);
   print_r(arr_foreach($a));
?>

2. Using the for loop, you can only traverse an array of numeric subscripts

<?php
   function arr_foreach($arr)
   {
      static $tmp=array();  

      for($i=0; $i<count($arr); $i++)
      {  
         if(is_array($arr[$i]))
         {  
            arr_foreach($arr[$i]);  
         }else{  
            $tmp[]=$arr[$i];  
         }  
      }  

      return $tmp;  
   } 
   // Call example 
   $a = array(1,array(3,array(5,6)),7);
   print_r(arr_foreach($a));
?>

3. Use while

/**
 *  Converts a multidimensional array to 1 Dimensional array 
 * @author echo
 * @link https://www.ofstack.com/
 * @param array $arr
 * @return array
 */
function ArrMd2Ud($arr) {
 # The numerical first 1 The element acts as a container for address assignment. 
 $ar_room = &$arr[key($arr)];
 # The first 1 Containers are not arrays 
 if (!is_array($ar_room)) {
  # Put it in an array 
  $ar_room = array($ar_room);
 }
 # Hands down 
 next($arr);
 # traverse 
 while (list($k, $v) = each($arr)) {
  # It's an array, it's recursive, it's not an array 
  $v = is_array($v) ? call_user_func(__FUNCTION__, $v) : array($v);
  # Recursive merger 
  $ar_room = array_merge_recursive($ar_room, $v);
  # Frees the array element of the current subscript 
  unset($arr[$k]);
 }
 return $ar_room;
}

Example calls:

$arr = array(1, 2, 3 => array(1, 2, 'ar' => array(1, 2 => array('a', 'b'))), array('ar' => array(3, 4)));
print_r(ArrMd2Ud($arr));

Output:

Array
(
    [0] => 1
    [1] => 2
    [2] => 1
    [3] => 2
    [4] => 1
    [5] => a
    [6] => b
    [7] => 3
    [8] => 4
)