PHP：数组实现笛卡尔积

<?php
$arr1 = [];
$arr2 = [
  ['淡', '深'],
];
$arr3 = [
  ['淡', '深'],
  ['红', '黄', '蓝']
];
function descartes($arr)
{
  // 取出第一个
  $result = array_shift($arr);
  //  直到没有数据
  while ($arr2 = array_shift($arr)) {
    $arr1 = $result;
    // 重置
    $result = [];
    foreach ($arr1 as $item1) {
      foreach ($arr2 as $item2) {
        $result[] = "${item1}${item2}";
      }
    }
  }
  return $result;
}
echo json_encode(descartes($arr1), JSON_UNESCAPED_UNICODE) . PHP_EOL;
// null
echo json_encode(descartes($arr2), JSON_UNESCAPED_UNICODE) . PHP_EOL;
// ["淡","深"]
echo json_encode(descartes($arr3), JSON_UNESCAPED_UNICODE) . PHP_EOL;
// ["淡红","淡黄","淡蓝","深红","深黄","深蓝"]