Im stuck on this for quite some time. I have to work with identifiers that are always 4 characters long and consist of 0-9a-z. An example would be iulz. So the range would be something like:

我在这个问题上坚持了很长一段时间。我必须使用总是4个字符长并且由0-9a-z组成的标识符。iulz就是一个例子。所以范围大概是：

0000 0001 0002 ... 0009 000a 000b ... 000z 0010 ... zzzz

0000 0001 0002。。。0009 000a 000b。。。000z 0010。。。zzzz

So in the case of my example, iulz I know that the ID before that would be iuly and after that ium0. But i cannot seem to work out how to calculate this. The functions hexdec and dechex would help me if they would include the whole alphabet.

所以在我的例子中，iulz，我知道在那之前的ID是iuly，在那之后是iu0。但我似乎不知道该怎么计算。如果函数hexdec和dechex包括整个字母表，它们会对我有所帮助。

My first try was to make a logical list with some for loops but this takes an impossible amount of memory. I am looking for a way to make the following two functions:

我的第一次尝试是用一些for循环制作一个逻辑列表，但这需要大量的内存。我正在寻找一种实现以下两个功能的方法：

function getNext(string $id) : string { ... }
function getPrev(string $id) : string { ... }
$this->getNext('iulz') // returns 'ium0'
$this->getPrev('iulz') // returns 'iuly'

Can anyone get me on the right track here? Thnxz!

有人能让我走上正轨吗？Thnxz！

Your encoding scheme contains 36 symbols (digits from 0 to 9 and letters from a to z), so a possible approach is to implement a math with Base36. PHP supports base_convert() function to convert a number between arbitrary bases.

您的编码方案包含36个符号（数字从0到9，字母从a到z），因此一种可能的方法是使用Base36实现数学运算。PHP支持base_convert（）函数在任意基数之间转换数字。

<?php

function getNext($id) {
    $n = base_convert($id, 36, 10);
    $n = $n + 1;
    $n = base_convert($n, 10, 36);
    $n = str_pad($n, 4, '0', STR_PAD_LEFT);
    return $n;
}

function getPrev($id) {
    $n = base_convert($id, 36, 10);
    $n = $n - 1;
    $n = base_convert($n, 10, 36);
    $n = str_pad($n, 4, '0', STR_PAD_LEFT);
    return $n;
}

$id = 'iulz';
echo "ID: " . $id . ", next ID: " . getNext($id) . ", previous ID: " . getPrev($id) . ".";

?>

Result:

结果：

ID: iulz, next ID: ium0, previous ID: iuly.

As an additional note, you need to define the expected behaviour of the functions, if the $id is outside the [0000 .. zzzz] range.

另外需要注意的是，如果$id在[000.zzzz]范围之外，则需要定义函数的预期行为。

function previousId(string $alphabet, string $id)
{
  $length = strlen($id);
  $first = $alphabet[0];
  $last = $alphabet[strlen($alphabet) - 1];
  if ($id === str_repeat($first, $length)) {
    return '––––';
  }
  for ($len = $length - 1; $len >= 0; $len--) {
    if (str_ends_with($id, str_repeat($first, $len))) {
      $pos = $length - $len - 1;
      $char = $alphabet[strpos($alphabet, $id[$pos]) - 1];
      return substr($id, 0, $pos) . $char . str_repeat($last, $len);
    }
  }
}

function nextId(string $alphabet, string $id)
{
  $length = strlen($id);
  $first = $alphabet[0];
  $last = $alphabet[strlen($alphabet) - 1];
  if ($id === str_repeat($last, $length)) {
    return '––––';
  }
  for ($len = $length - 1; $len >= 0; $len--) {
    if (str_ends_with($id, str_repeat($last, $len))) {
      $pos = $length - $len - 1;
      $char = $alphabet[strpos($alphabet, $id[$pos]) + 1];
      return substr($id, 0, $pos) . $char . str_repeat($first, $len);
    }
  }
}

These two functions take an arbitrary alphabet (of unique characters). The id can be of any length smaller than the length of the alphabet string. Both error condition could and should be added to these function.

这两个函数采用（唯一字符的）任意字母表。id可以是比字母字符串的长度小的任何长度。这两个错误条件都可以而且应该添加到这些函数中。

If there is no preceding value for previousId the function currently returns the message '----' for illustration purposes. Possible real values: false, null, wrap around to the last value ('zzzz'), or maybe throw a RuntimeException or test the input value before calling previousId. Same applies for a call of nextId with an id of 'zzzz', which could return '0000', or false, or null, etc.

如果previousId没有前面的值，则函数当前会返回消息“----”以供说明。可能的实际值：false、null、换行到最后一个值（“zzzz”），或者可能引发RuntimeException或在调用previousId之前测试输入值。同样适用于id为“zzzz”的nextId调用，该调用可能返回“0000”、false或null等。

$alphabet = '0123456789abcdefghijklmnopqrstuvwxyz';
$id = 'iulz';

echo previousId($alphabet, $id) . " < $id > " . nextId($alphabet, $id) . "\n";

// Outputs: iuly < iulz > ium0