我试图了解如何回溯工作在php。我明白的理论，但我不明白的代码。
这是一个简单的例子：

function permute($nums)
{
    $result = array();
    backTracking($nums, count($nums), 0, array(), $result);
    return $result;
}

function backTracking($nums, $numsLength, $startIndex, $permutation, &$result)
{

    if (count($permutation) == $numsLength) {
        array_push($result, $permutation);
        print_r("return" . "\n");
        return;
    }

    for ($i = $startIndex; $i < $numsLength; $i++) {
        if (!in_array($nums[$i], $permutation)) {
            array_push($permutation, $nums[$i]);
            print_r($i . "\n");
            print_r($permutation);
            backTracking($nums, $numsLength, 0, $permutation, $result);
            print_r('pop : ' . array_pop($permutation) . "\n");
        }
    }
}

$nums = [1, 2, 3];
var_dump(permute($nums));

字符串
结果：

0
Array
(
    [0] => 1
)
1
Array
(
    [0] => 1
    [1] => 2
)
2
Array
(
    [0] => 1
    [1] => 2
    [2] => 3
)
return
pop : 3
pop : 2
2
Array
(
    [0] => 1
    [1] => 3
)
1
Array
(
    [0] => 1
    [1] => 3
    [2] => 2
)
return
pop : 2
pop : 3
pop : 1
1
Array
(
    [0] => 2
)
0
Array
(
    [0] => 2
    [1] => 1
)
2
Array
(
    [0] => 2
    [1] => 1
    [2] => 3
)
return
pop : 3
pop : 1
2
Array
(
    [0] => 2
    [1] => 3
)
0
Array
(
    [0] => 2
    [1] => 3
    [2] => 1
)
return
pop : 1
pop : 3
pop : 2
2
Array
(
    [0] => 3
)
0
Array
(
    [0] => 3
    [1] => 1
)
1
Array
(
    [0] => 3
    [1] => 1
    [2] => 2
)
return
pop : 2
pop : 1
1
Array
(
    [0] => 3
    [1] => 2
)
0
Array
(
    [0] => 3
    [1] => 2
    [2] => 1
)
return
pop : 1
pop : 2
pop : 3
array(6) {
  [0]=>
  array(3) {
    [0]=>
    int(1)
    [1]=>
    int(2)
    [2]=>
    int(3)
  }
  [1]=>
  array(3) {
    [0]=>
    int(1)
    [1]=>
    int(3)
    [2]=>
    int(2)
  }
  [2]=>
  array(3) {
    [0]=>
    int(2)
    [1]=>
    int(1)
    [2]=>
    int(3)
  }
  [3]=>
  array(3) {
    [0]=>
    int(2)
    [1]=>
    int(3)
    [2]=>
    int(1)
  }
  [4]=>
  array(3) {
    [0]=>
    int(3)
    [1]=>
    int(1)
    [2]=>
    int(2)
  }
  [5]=>
  array(3) {
    [0]=>
    int(3)
    [1]=>
    int(2)
    [2]=>
    int(1)
  }
}

型
也许这是一个简单的问题，但为什么函数循环两次或三次弹出？它取决于执行的帧数？然后;索引取决于什么？
谢谢你们！