PHP uses recursive algorithm to traverse an array infinitely

<?php
//无限遍历数组
$a1 = array("a", "b", "c"); //一维数组
$a2 = array(array(21, 3, 6), array("a", "b", "c")); //二维数组
$a3 = array(array(array(5, 55), 4, 444), 2, 7, 6, 8, array("w", "d", array(3, 2, "a"), "s")); //多维不规则数组
function fun($a) {
  foreach ($a as $val) {
    if (is_array($val)) { //如果键值是数组，则进行函数递归调用
      fun($val);
    } else { // 如果键值是数值，则进行输出
      echo "$val<br />";
    } //end if
     
  } //end foreach
   
} //end fun
//fun($a1);
//fun($a2);
fun($a3);
?>

5
55
4
444
2
7
6
8
w
d
3
2
a
s