php 一维数组变多维

将下面的数组处理成下图的样子

$arr = ['a', 'b', 'c', 'd','e','f'];

<?php$arr = ['a', 'b', 'c', 'd','e','f'];$x = [];$y = &$x; $value = 1234;while ($key = current($arr)) {    $y[$key] = [];    $y = &$y[$key];    next($arr);    //echo $key;}$y = $value;print_r($x);?>