php - 如何在php中打印所有可能的字符串(39个非整数字符串)混合符号输出站点地图

我的用户帐户规则是限制20个符号，可以是字母、句点、下划线和数字。
第一季度。如何打印所有可能的字符串？下面是我的代码我找不到混合不同符号的方法
问题2这是正确的方式打印网站网站地图，因为它是39个非百万字符串，我发现https://stackoverflow.com/a/1099421但我需要知道更多的细节，了解通常人们如何做？

$letters = array('a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z');
$period = array('.');
$underscore = array('_');
$numbers = array('0','1','2','3','4','5','6','7','8','9');

for ($i=0; $i < count($letters); $i++) {
  for ($ii=0; $ii < count($period); $ii++) {
    for ($iii=0; $iii < count($underscore); $iii++) {
      for ($iiii=0; $iiii < count($numbers); $iiii++) {
        // echo $numbers[$iiii]+$letters[$i]+ ...
      }
    }
  }
}

编辑：
基于下面的答案，我试图创建一个XML站点放大器，但它只循环一个符号长度？

$domDocument = new DOMDocument('1.0', 'UTF-8');

$domElementUrlSet = $domDocument->createElement('urlset');
$domElementUrlSet->appendChild(
  new DomAttr('xmlns', 'http://www.sitemaps.org/schemas/sitemap/0.9')
);

$domDocument->appendChild($domElementUrlSet);



$chars = array(
    'a','b','c','d','e','f','g','h',
    'i','j','k','l','m','n','o','p',
    'q','r','s','t','u','v','w','x',
    'y','z','.','_','0','1','2','3',
    '4','5','6','7','8','9'
);
$length = 2;
$charsLength = count($chars);
$current = array_fill(0, $length, -1);
$end = array_fill(0, $length, $charsLength - 1);

while ($current != $end) {
    // increment current state
    $n = $length;
    while ($n-- >= 0) {
        $current[$n]++;
        if ($current[$n] == $charsLength) {
            $current[$n] = 0;
        } else {
            break;
        }
    }
    // print string
    for ($i=0; $i < $length; $i++) {
      if ($current[$i] >= 0) {
        // echo $chars[$current[$i]];

        $url = $domDocument->createElement('url');
        $url->appendChild( $loc = $domDocument->createElement('loc', 'http://www.example.com/'.$chars[$current[$i]]) );

        $domElementUrlSet->appendChild($url);
      }
    }
    // echo PHP_EOL;
}

echo $domDocument->saveXML();

最佳答案

这项任务相当琐碎您不需要单独的集合字符，只需要一个字母表并遍历所有可能的组合：

<?php
$chars = array(
    'a','b','c','d','e','f','g','h',
    'i','j','k','l','m','n','o','p',
    'q','r','s','t','u','v','w','x',
    'y','z','.','_','0','1','2','3',
    '4','5','6','7','8','9'
);
$length = 20;
$charsLength = count($chars);
$current = array_fill(0, $length, -1);
$end = array_fill(0, $length, $charsLength - 1);

while ($current != $end) {
    // increment current state
    $n = $length;
    while ($n-- >= 0) {
        $current[$n]++;
        if ($current[$n] == $charsLength) {
            $current[$n] = 0;
        } else {
            break;
        }
    }
    // print string
    for ($i=0; $i < $length; $i++) {
        if ($current[$i] >= 0) echo $chars[$current[$i]];
    }
    echo PHP_EOL;
}