PHP实现给定一列字符，生成指定长度的所有可能组合示例

<?php
function de($len, $pos = 0) {
  static $bit = [];
  static $source = ['a', 'b', 'c', 'd', 'e'];/*[0, 1, 2, 3, 4, 5, 6, 7, 8, 9];*/
  $pos++;
  for($i = 0; $i < count($source); $i++) {
    $bit[$pos] = $source[$i];
    if ($pos < $len) {
      de($len, $pos);
    } else {
      echo implode('', $bit)."\n";
    }
  }
}